Both are useful tools but CMC is more popular than Coingecko!
That has been my experience as well, tho I do use both sites for information.
Because they don't have the same standard.
That's probably the best answer to op's question. One might ask why Binance is so much easier to use than an exchange with an awful gui like southxchange...and the answer is that some people know what they're doing and how to make a better website than other people. It's a difference of standards, interest, talent, and probably a whole host of other factors.
And honestly if coingecko disappeared overnight, I'd be completely fine just using coinmarketcap from then on. Their site has pretty much all the info I usually look for.