Meaning: lots of traders are from Europe and SEPA zone (especially from Germany, UK, Fance, Netherlands, Spain, Italy, Belgium). SEPA payments are fast enough, liquidity is also good.
Why the don't you, Europeans, trade in /EUR markets?
They do, they just use it for cashing out and not trading. We are quite fine with dollar trading to be honest with you, trading doesn't require cashing out or even euro at all, that is when you want to get out of the market totally.
The British have their own websites in the country that allows you to turn bitcoin into pound and send it to your bank, the french have their own exchanges that allow you to turn your bitcoins to euro and send it to bank, Spanish have their own, I can continue giving examples but you understand the logic. So, Europeans are fine with trading whatever , they can buy bitcoin and use that bitcoin to buy and sell altcoins and in the end if they want to get out there are ways to do that and send it to your bank anyway. So, trading in Euros doesn't really posses a need for it.