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.