HTML is all about providing structure and meaning to content, and this should be its sole function.
However, there are assorted tags that were added in the past by stupid browser developers, which violate this basic principle.
At some point, people came to their senses (partially, at least), and started (trying to) agree upon a specification for writing correct HTML, so they deprecated some of these tags.
However, the damage has already been done - too many people have learnt HTML as "put tags around stuff and stuff happens", rather than understanding the (important and useful) distinction between semantics and presentation, and those people have gone on to write books and tutorial, teaching other people the wrong way to write HTML, who themselves have done the same, and we've ended up with a right old mess.
center tag should never have existed, because people should have pointed out it's not 'real' HTML, and told the browser makers to go back and do it properly.
And for doing it properly... alignment is of course a presentation issue - applying layout and formatting to (structured) content - and the language to use for that is CSS.
However, yet again, stupid browser developers screwed up early CSS - and in many cases people creating web pages had to use incorrect HTML because the CSS simply didn't work.
These days, we're starting to get some useful functionality added to CSS, but there's still a lot that isn't implemented, and quite a few things that could have been implemented in far superior ways, but are now standardised so we're stuck with the old ways.
If stylesheets had been implemented by sensible people, you'd have all the controls you have in any decent office software - for aligning blocks and text, it'd work horizontally, vertically, upside-down, rotated, and so on - and it wouldn't be done in the stupid way of setting left/right margins to auto.
Yet again, those stupid browser developers didn't know what they were doing. They picked an utterly inaccurate/invalid name (for marketing reasons), and again riddled a potentially great and powerful language with flaws, lack of functionality, and created something that was (until recently) hated by many people.
So yeah, to summarise all that, the ultimate answer is because browser makers were stupid. :)