I tried Mouseguard years ago and didn’t care for it very much. It was a one-off thing. Now I really like Burning Wheel. Yes there are differences, but in the context of a one-off, they’re close enough.
So I second the “give it another go” thought. I was expecting something very different. I approached the experience looking for the same things I was used to from other RPG systems. And so I missed many of the good things about the game.
That said, no one system is good for everybody. Could be that these players just don’t want what Burning Wheel has to offer.