Back lever is much stronger than front lever because of a couple reasons:
~Lats are in a much more mechanically efficient position/angle of pulling. Since they insert on the thoracolumbar fascia on the back, with your arms in front for the front lever they have to basically wrap around your body and are not getting full force production out of them. In the back, the mechanical angle is much more efficient.
~We are stronger in flexion in general than extension. Even though the arms are behind the body, the scapula & humerus and all of the muscles that insert/originate from it are much more efficient (just like the example above) in performing movements that are generally in front of our body.
~lower back, glutes, legs contract harder and thus get more full body tension from back lever than in front lever with the abs.
~Often pulling work is limited by weakness in the external rotators, posterior delts, scapular retractors, etc. A lot of stalled front lever progress can be mitigated if you focus on those specific areas to bring them up to par.