Recently I read a lot about noSQL DBMSs. I understand CAP theorem, ACID rules, BASE rules and the basic theory. But didn't find any resources on why is noSQL scalable more easily than RDBMS (e.g. in case of a system that requires lots of DB servers)?

I guess that keeping constraints and foreign keys cost resources and when a DBMS is distributed, it is a lot more complicated. But I expect there's a lot more than this.

Can someone please explain how noSQL/SQL affects scalability?

noSQL databases give up a massive amount of functionality that a SQL database gives you by it's very nature. 

Things like automatic enforcement of referential integrity, transactions, etc. These are all things that are very handy to have for some problems, and which require some interesting techniques to scale outside of a single server (think about what happens if you need to lock two tables for an atomic transaction, and they are on different servers!).

noSQL databases don't have all that. If you need that stuff, you need to do it yourself, but if you DON'T need it (and there are a lot of applications that don't), then boy howdy are you in luck. The DB doesn't have to do all of these complex operations and locking across much of the dataset, so it's really easy to partition the thing across many servers/disks/whatever and have it work really fast.
