If there was a wider range of support for XMR OUTSIDE of Polo, you'd see those exchanges having a much higher volume. For weeks XMR has been the #1 volume maker for Polo. Without it, the volume on Polo would have been next to nothing. Show me another high ranking coin that has more volume on Poloniex than any other exchange. I don't think there is one. Polo has controlled the XMR market ever since it was added there.
I frankly don't understand your obsession with Polo. Polo did the work to integrate the different cryptonote API before the other exchanges and for quite a while it was the only major exchange. The nature of the exchange business is that people want to trade where there is the most liquidity, so this is naturally self-reinforcing, and trading tends to stay concentrated wherever it already is (absent some catastrophic failure forcing a move). That is why most of the coins have a dominant exchange, as I demonstrated (and most of them are more concentrated on their dominant exchange than XMR). It pretty much has to be Polo for XMR because Polo got established first and hasn't had a catastrophic failure. I fail to see how which exchange happens to be dominant for a particular coin even matters.