Those direct flights were always full to bursting, in both directions. No reason why they should have discontinued them.
Rather than routing through IAH, DEN is just as good except in winter. You get main line planes and upgrades can be easier.
Passenger loads have nothing to do with profitability. It is all about PRASM. The ease of freebie UG's would seem to point to not much paid F and that is a big money-maker. The converse is true xDFW. All that inbound traffic from Europe connecting onwards creates relatively high PRASM traffic in both Y & F.
This is not solely a UA issue. Rather it is a legacy at non-hub issue where the station is another carrier's hub. I can't speak for direct flights, but UA is only one of four carriers with nonstop service with AA, of course, offering what amounts to hourly service