Thats the reason. Moyes is very reluctant to change a winning team, hes always done that in the past, a few weeks ago when it needed changing he wasn't available and now joey and wier seem to be doing the biz
I think that what you have said is part of the issue - if its not sick, dont give it medicine.
ALso - with him on the bench - its made Weir scared of losing his place - competition has raised his game.
The other reason, i think, is that he has not yet really had enough English football and its difficult to bring in a foreign player in such a key role.
Kroldrup is mainly used to soppy italian strikers who jog and dont run. Coming up against the likes of Yakubu would be difficult for him to cope with.
I thought that it was a mistake when Moyes did not play him in the Carling cup - thought that the cup would have been a good platform for his proper introduction to English football...
Be patient - he will get his chance - its only a matter of time - Weir is too old and too slow for the CB role - in my humble opinion (Although he has had two good games lately!)