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.
i think with ten different coins 85% of the market is covered
I doubt they'll implement too many coins. top 10 sounds reasonable enough.
top 5 is good enough. BTC, ETH, LTC, DASH, NEO such as
I think in this case, they should apply "Pareto rule" as known as "80/20 rule". They should list the coins which cover about 80% total market cap such as BTC, ETH, Ripple, BCH, LTC, DASH, and so on...