Back flips are easier than front flips for us

Human beings tend to put more strength when open their hip joint from lean back posture than lean forward.
Since back muscles is stronger than an abdominal muscle, the power of leaning back is stronger than forward.

Backward rotation occurs when your hip joint is opened, so back flip is generally said to be easy and improved than front flip in human history.
Type 2 standing front flip is less efficient in respect of power of pushing the ground by straighten hip joint, type 1 is less efficient too in respect of the direction than back flip.