The build up of the US army from a small, backwards, underfunded, isolationist peace-time army in 1939 to a six million person world conquering colossus in 1945 is one of the under-appreciated triumphs of WWII. Much can be attributed to the cadre of professional, forward thinking officers like George Marshall the US maintained. When it comes to tanks, it was a devotion to automotive quality, focusing on a single design, and "leapfrogging" the enemy in guns and armor.
Let me lay out some ground rules. This answer is in the context of 1942, later war tank developments do not apply. The US tanks will only be compared to their intended and actual opponents, Soviet tanks may be great but the M4 wasn't designed to fight them.
In WWII the main purpose of tanks was not to fight other tanks, it was to support the infantry, either in the infantry support role (infantry / heavy tanks like the Matilda or early Panzer IV) or cavalry role (medium and light / cavalry / cruiser tanks like the Crusader). Good WWII tank design considered these elements.
- Speed of manufacture
- Ease of manufacture (simpler parts can be made by more factories)
- Mechanical reliability
- Ease of maintenance
- Fuel consumption
- Road performance
- Cross-country performance
- Ergonomics (how well can the crew can operate the tank)
- Armament vs infantry (machine guns & high explosive rounds)
- Armament vs armor
- Armor vs high explosive (artillery)
- Armor vs kinetic (anti-tank guns)
This can explain a lot of apparently odd decisions on the part of US tank designers. They liked the 75mm gun because it fired an excellent high explosive round, very effective against infantry, whereas the 76mm gun with better anti-tank performance had a less effective HE round. They considered maneuverability and mechanical reliability more important than heavy armor and anti-tank performance, because the US army was a very aggressive and wanted to make sweeping attacks and avoid slogging matches.
On to the question. The US had a huge advantage over Britain and Germany in 1940-1942, an enormous, underutilized and experienced automotive industrial base. This meant US tank designs would be mechanically well-designed and reliable. Better engines and transmissions can carry more armor at better speed. Their excess of productivity meant the designs could also be lavish, especially relative to the extremely overstretched British.
The US had another advantage. Prior to the war they had almost no tanks to speak of and a very small army. This meant they had no prior stock of parts and ammunition to consider. They didn't have to rush half-finished designs into combat either. They could sit back and watch the development in Europe and stock their new army with the latest and the greatest. While the US Army had little experience in armored warfare, and budgets were miniscule, they knew a war was brewing and had laid the ground work for armored tactics and design.
Rather than working on a whole bunch of different tank chassis (the Germans had at least six in production in 1942, the British were probably as bad) they focused on one. Starting in 1935, they gained experience with the M2 Light Tank, which developed into the M2 Medium Tank or "the tank that proved you can have too many machine guns". But it was fast, reliable, and pretty easy to build.
When a layman considers a tank, they see the gun and armor and that's it. It cannot be overstated how important it is for a tank to have a good drive-train, and how many otherwise excellent tanks were crippled by bad transmissions and weak engines overloaded with too much armor. Tank engines have to move 30-70 tons at 30mph, and their transmissions have to transmit 400-700 horsepower at high torque. It doesn't matter how good your tank is if it breaks down or gets stuck in the mud every 50 miles. It doesn't matter how thick your front armor is if I can drive around behind you. The Tiger never solved their drive-train problems. The early Panther production was crippled by them, in the Battle of Kursk nearly as many were lost to breakdowns as to enemy action. The KV/IS series of tanks was, in some ways, Soviet engineers learning to make decent transmissions.
The Americans avoided all that by using the same Wright/Continental radial aircraft engine on the M2, M3 and early M4. It was a compact, reliable, existing engine, with an existing production line. It had the torque and power necessary to drive a well armored tank at a good speed giving it a better power/weight ratio than the PzIII or IV). When they couldn't produce enough engines, they grabbed another existing aircraft engine, the Ford GAA V8. This had the trade-off of using gasoline, more flammable than diesel, but the Americans thought the extra power was worth it.
The invasion of Poland and France showed the M2 was under-gunned, under-armored, poorly laid out, and too tall, but the British army, having littered France with much of their heavy equipment, needed tanks RIGHT NOW, so the US went about designing an expedient tank. It was known that the 37mm gun was inadequate. Everyone else was incrementally upgunning, the US wanted to leapfrog everyone with a 75mm gun (a huge gun in 1940, the Soviets had the same idea earlier with the T-34), but designing a new turret takes a long time. The M3 Lee/Grant used the same chassis, drive train and 37mm turret as the M2 Medium taking advantage of a known good design and existing production lines. It deleted most of the machine guns, added some armor, and, most obviously, stuck the 75mm out the side of the tank.
The M3 was awkward, but it did the job, and it was quite a surprise at the Battle Of Gazala. The 75mm was far superior to the 2 pounders the British were fielding and could out-range most of what the Germans and Italians had. The best the Germans had was the Panzer III with a 50mm gun and thin armor. Its armor was flawed (the rivets had a tendency to become projectiles), but adequate. It was also fast, as fast or faster than most tanks at the time. In some ways, the M3 was the first Main Battle Tank (ie. it could cover both the infantry and cruiser tank role) fielded by the Western Allies (the Soviets already had the T-34).
With the M3 in production, and the British reporting painful lessons back, the US got down to the business of designing the M4. They chose the simplest. It kept the good parts of the M3 (suspension, transmission, chassis and 75mm gun), fixed the layout, and added a new turret. Like the M3, the M4 was designed to do it all. It had the armor and HE round to support infantry, it had the speed, reliability and cross-country performance for breakthroughs and cavalry dashes, and at the time it had the gun and armor to defeat anything in the Axis arsenal. This was all true until it met the Tiger (and later Panther).
There you have it. A cadre of professional, pre-war officers made sure to keep the US Army up to date and in practice with tanks despite a miniscule budget. They used their extensive automotive industry and experience to design a fast, reliable drive-train and chassis which they continually improved and upgraded. They observed events in Europe (particularly the battles of Poland and France) and altered their plans accordingly, only committing to their final design (the M4) in 1941.