Several here have talked of the rim being bent but not at length. A new, good rim can be laid on a pane of glass and touch all around the rim. The wheelbuilder's task is to take that rim and arrange it around the hub so it is centered and keeps that now invisible pane. Truing of that rim later is simply returning the rim to its natural state.
But, if the rim have been bent; either from a blow inward (hitting a pothole, say) or to the side (crashes and non-riding accidents to the bike or wheel) the game is different. Now the wheelbuilder has to either juggle spoke tensions to force the rim to a shape it will no longer take naturally or (violently) re-damage the rim in the opposite direction.
In the old days, with the much "softer" aluminums, re-bending the rim was common practice. Good wheel builders had the tools and skills. Now, the much stronger, harder aluminums take far less kindly that second bend. Quality wheelbuilders will not do it. Lesser damage can be tweaked to acceptably true with unequal spoke tensions and often made to be ridable wheels but they will never be great wheels. Different wheelbuilders have different levels of willingness to do this. The fact that one builder will and a another will not does not necessarily make the one who will a better wheelbuilder, just one who is more willing to make an unridable wheel an OK ridable wheel that will never be a good wheel.