In the original comic, I don’t think there was any specific reason why each one received different powers. One could argue that different DNA would be affected differently by radiation. Just like how some people get cancer from sunbathing and some don’t. I think Stan Lee was trying to represent each of the traditional elements: fire, air, water and earth.
In more recent retellings of the origin of the Fantastic Four, their powers have been tied in to their personality types:
Johnny Storm – Hot tempered; turned into Human Torch
Susan Storm – Timid and shy; turned into Invisible Woman
Reed Richards – “Outside-the-box” thinker; turned into Mr. Fantastic
Benjamin Grimm – Streetwise tough-guy; turned into the Thing
I also recall one retelling that had them gain their abilities based on what was happening to them in the experimental spaceship.
Johnny – a fire breaking out next to him caused him to be resilient to flames
Susan – trying to stop the collapsed hull from crushing her manifested into force-field powers
Reed – trying to stretch out and reach the injured Sue gave him the ability to stretch his body
Ben – straining with all his might to keep the ship under control gave him an increase in strength
@XOIIO One must know his enemies very well in order to crush them.