As title, I am curious why most of the aircraft model are either 1/48 or 1/72, which prevent me by combing them with my 1/35 figures for making dioramas. Any clues?

1/35 scale was started by Tamiya because they wanted to squeeze in batteries and a motor in every tank they made. As market leaders everyone started to follow them.

The traditional scale was 1/32, which you can find some old Airfix or Monogram AFVs in.

IIRC 1/35 was the smallest scale Tamiya could fit RC equipment into the hull of a Tiger I without having to distort the original shape of the tank.

You will find some helicopters in 1:35 scale but as far as aircraft go....they are in 1:32 scale
and you can find quite a selection.

As far as adding your 1:35 figures to a diorama with 1:32
aircraft, the difference is so little that you can get away
with it.

Suggestion: Use your 1:35 scale stuff mostly in the foreground
and the 1:32 in the background...perspective, when viewing, will
bring it all together.

Cheers,
Joe

This site lists a few airplanes as well as helicopters in 1/35 scale. Unfortunately almost all "Sold Out"

http://www.1999.co.jp/eng/list/231/0/1

Actually for proper perspective the 1/32 scale should be in the foreground. However from a practical standpoint, it probably really doesn't matter because most people are not familiar with the size of airplanes compared to vehicles. And as you said the scales are close.