Probably because of merchandising= Very yes.
Somehow, Cars merchandise does sell. They just overestimated the second film and overproduced all the merchandise. Everywhere I go, I see it either on clearance or on sale, only for it to be repegged.
Planes has less appeal. I mean, kids love playing with die cast cars, so Cars had that instant fanbase... planes would be a niche market for kids. If they did a trains movie instead (despite the fact that Thomas and Chuggington kinda have that all sewed up), it would probably be a bigger draw.
I just want it to flop so we don't have to suffer through Funiculars and Vespas. And having Disney become the "upstart vehicle learns it's place" factory.