Why only 10? They can add at last top 50 coins... More coins - more users on their platform.
Because it would be a mess maybe, the more coins they add the more resources they have to spend (money, time etc...). I think working, secure and stable platform = more users (not more coins)
and honestly, there are only a couple of coins which are very stable and have a good volume and wont hit hard due to the volatility of bitcoin very much, such as ltc and so on.