Poloniex used to have a huge number of coins, but in about March they culled a lot of them as they weren't doing a lot of trade and were just gumming up their engine. When a trading engine gets to a certain size with lots of users, you can't afford the small coins, because they slow the whole thing down.

Kraken has just removed a whole bunch of pairs too, to improve the performance of their engine.

With Bittrex - they do list coins, but they also aggresively delist them if the volume of trades falls in a two week period.