Several edits made; the important ones are bolded. Also, NB: I am essentially retelling my own experiences, here, and then generalizing them. So please add grains of salt where necessary.
MetaPost vs TikZ: no discrimination, just different popularity. As far as I can tell, the languages are about equally easy to use. So what, then, made TikZ so popular?
Firstly, TikZ has an examplary manual. It is good-looking, which makes it attractive (in the literal sense). It has an introductory tone — and it covers every feature of the programme. It has examples — it has examples for everything. It describes features general purpose and usage — and it describes their behaviour and implementation in-depth. It is my belief this has justly made it famous;
and MetaPost, frankly, lacks such a manual.
Edit: @mbork rightly points out in a comment that the MetaFun manual is pretty good; and I might add John Hobby's MetaPost manual to that, too. All the same, I think that this is a case of ‘good’ getting outclassed by ‘outstanding’. Here are links to the manuals, so one can judge for oneself.
Secondly, and less importantly, Till Tantau has also written the very popular beamer package. Like TikZ, beamer is very user-friendly, and has an excellent manual. If people already know and like beamer, they will be predisposed to discover or like TikZ, because they know they can trust its author to write a user-friendly package with an outstanding manual.
(This paragraph is edited) MetaPost has MetaFont, but creating fonts is a small niche compared to creating presentations. It is also very nicely integrated with ConTeXt, and indeed I think that ConTeXt users (like me) use it more than TikZ (I know I do); but I don't think that affects the fashions among LaTeX users very much.
MetaPost is a lovely language, but I don't believe it has a manual that is anywhere near as good. So, when MetaPost was still the more popular drawing package, TikZ was nonetheless easier to recommend to newbies; and by now, TikZ is itself the most popular drawing package, which only strengthens its position.
So, yeah. It's not the quality of the languages -- TikZ is as good as MetaPost, and vice versa. It's not conspiracy, of course. It is, I believe, TikZ's spectacular manual.