Who really knows
why this is? No one, I suspect, because there are too many variables involved.
Back in the day, in certain instances, kits were made to fit a box. Hence the term "box scale".
One explanation I can think of is that maybe at one point certain manufacturers started releasing kits in certain scales only because of the success of other manufacturers' kits in those scales. (Aurora may be responsible, in whole or in part, for the advent and resulting popularity of 1/48 aircraft) After a time, those scales became the norm and therefore the "accepted" scales for certain types of models.
In cases like Monogram's military vehicles, most of the kits we think of (Screamin' Mimi, Grant/Lee, Panzer IV (or is it III?) and the rest of those BIG boxed kits) are 1/32. Some of their stuff is 1/35, however. Now Monogram already had some aircraft kits out in 1/32 when they brought out the 1/32 armor. This was around the time that Shep Paine was doing a bunch of work for them (remember the "how-to" diorama booklets included in many kits at the time?). Perhaps they had some plans to try and cross market the two genres in the scale of 1/32? I dunno. I don't know why they did this, it's just a guess on my part. And there are other guesses in my head, but none so seemingly logical as this one.
![Tongue [:P]](/FSM/CS/emoticons/icon_smile_tongue.gif)
Anyhoo, I used to wonder about all this myself. I don't do it anymore. I'm no longer a rivet counter and if I'm going to do a large scale diorama which includes an aircraft and a target, er... tank or other military vehicle, then I'll just use a combo of 32nd aircraft and 35th armor. For me, it's close enuff.
Or I'll do it in 1/48. There's plenty of 48th military stuff around to do something like that.
Fade to Black...