1. Every contest is a "battle" between raw points (total QSOs) and multipliers. The station in question may feel it is likelier that they will get multipliers by sitting outside the US General Band.
2. Which contest? In some contests, you, the stateside operator, get no points for working fellow stateside. In others, it is maybe a multiplier only and one you are likely to get from (say) Canadians anyway. In such cases, it is an advantage to be outside of the General portion because US stations are marginal at best.
3. There may be no room in the General portion of the band. Unless you are a complete a**hole, or even if you are, what happens if you switch to a crowded band after your run on (say) 15 meters peters out? If you switch to 20 or 40 at just the wrong time of day, because it is the "least worst" option, then you may end up in a part of the band you'd rather not use just because you can't find even a relatively clear part on the General part of the band. You don't have the rest of your life to look, either.
These are just some of the ways someone can end up outside of the general portion of the bands.