According to Wikipedia
"An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases."
Explosions are not made of fire. Think of them like a balloon, and everyday item that has energy inside of it (Helium, Air, etc.). If you try to pump more air into the balloon without letting the air escape, there will be too much and it will pop. If you think of it like that, it should make more sense. It is not a fire type move because the "energy" (Air in the Balloon example) does not need to be fire. It is a physical move because you are causing an overflow in energy, making it all let out at once. The visual part of the explosion is the energy bursting out which is often the reaction we see. Example, Coke and Mentos Creates a small explosion. The physical reaction needs somewhere to go, and that is what you see happening. The Air from the popped balloon does the same, except you can't see them.
TL;DR Not all explosions are made of Fire, so why should they be fire type. It is a physical move because the energy caused by the explosion would be a real entity's energy.
Hope I helped and Made Sense. Sorry I went all Science Nerd there.
My brain connecting the dots and the Wikipedia article on explosions.
Edit: sumwun made a point that Coke and Mentos made a physical reaction but you knew what I ment (hopefully)