To become the top 1 exchanges, they need to have many different elements, not just deposit volume. but the game is like that, whoever owns it has the right to rule and others to follow. CZ didn't naturally spend $ 400 million to buy CMC, he already had a plan in place for these marketing.
CMC has new features, which I guess they evaluate on their own, but before it was really different as exchanges are only ranked based on its 24 hours trading volume, but due to some exchanges faking its volume, CMC are making their own evaluation by adding some factors to determine the overall ranking of a site.
https://coinmarketcap.com/rankings/exchanges/as you can see, they already have the following :
Web traffic factor
Average liquidity.