Because the trains will cycle between routes regularly, if not daily.
Off-norm liveries are fine, and perhaps they break up the monotony of the standard fleet.... but a little goes a long way. Pretty soon appliances such as doors and appliance covers get swapped out, things get dinged and need retouching. Doesn't take long for things to start looking motley.
They probably do improve marketability, but I wouldn't take that too far. The core table stakes for marketability remain timing, comfort, choice of trains, pricing, and service, etc. The variety exterior is is the table setting but not the meal itself.
- Paul
PS - And if you look at the Lumi and say, hey it's just like the old Turbo..... you probably aren't from the demographic that VIA most cares about, and needs to sell to.