2h weapons hit harder when using your skills.
Only benefit on 1h is faster attack speed and double crit damage gems.
Also the benefit from the archery passive is much better for 2h xbows and bows.
Those aren't the reasons why people aren't using 1h xbows.
The real reason is that for the same dps, they're a lot more expensive. And considering that you can equip quivers on 2h bows anyway, the difference is not that substantial. Dual-wield 1h xbows still wield the highest potential though.
Imagine if anyone can wield 2 handed weapons with shields. That's what you get with the situation on 1h xbows.