I don't have a good explanation, but since I'm American, I'm just going to blame the metric system and let everyone else over here nod along with me
I'm guessing it has to do with the level of detail. Planes, typically being larger than tanks, can show off about as much detail at 1/48 as a tank can at 1/32 would be my guess.
I never could figure out what the designer of the M577 was thinking. It may look cool but it has practically no ground clearance. It might kick ass on undamaged pavement but off road even a few big rocks or a small stream would cause it to get stuck.
Planes is probably due to size and accommodating people's shelf space. I think 1/48 Armor is relatively 'new' in scale modeling time, i.e. a lot of kits started coming out a couple decades ago (Tamiya), think it was because they figured modelers wanted to do diorama's mixing the two. When you were a kid and wanted to 'play' with your models 1/72 was the way to go!