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 no one said, they will stick to the 10 different cryptocurrencies forever. It's just a number of different options they want to offer at the beginning. Who knows, maybe a new promising altcoin will emerge in 2018. As soon as everything is working properly they can start to think about adding new ones.
Definitely they will go with the flow of popular coins on which they should focus to avoid unnecessary work being done on the platform. Who knows, maybe in the future noone will be able to compete with top 5 coins and they will offer just those, or some new coins will surpass them.