I don't know if this is correct, just a spur of the moment thought, but this might be the reason front tumbling is harder.
After practicing front stuffs one time, my hamstrings were a little sore the next day. It hit me. When we get a running start or face forward from a skill and rebound out of that... instead of blocking by pushing the floor until our force overcomes gravity and going up, do we jump by PULLING and redirecting the force instead of pushing against it? The hamstrings are weaker than the quads (this theory supports why triple backs are possible and triple fronts are not), so that would explain why it's harder to tumble forwards.
Also, when tumbling backwards, you bend down slightly and lift up with your legs, back, and arms, then use your abs to pull together (lets say a tuck). In forward tumbling, there is no back involvement in tucks. It's simply your abs doing the work.
I wrote down more stuff as I thought about it, but I'm not going to type it right now... too lazy to go find the paper ![]()