I think the reason may be a bit more nuanced...John Lasseter and Ed Catmull have authority over all the animation groups at Disney. Planes was done by the DisneyToon group rather than the Pixar group.
In addition the original plan was to release Planes as a DVD movie, similar to the Tinker Bell movies also produced by the DisneyToon group, but Disney made a decision to release in theaters.
It's not a sequel exactly but a movie set in the same world where transportation is "alive." There is a Tow Mater short "Air Mater" that introduced Skipper, one of the characters in Planes.