Why can't I push up into urdhva dhanurasana?

The most common reason that I come across is tension in the hip flexors that is restricting the movement of the pelvis. That means you have to work even harder to push past that to push up in urdhva dhanurasana (backbend). To test to see if that is the case for you, do some backbending prep postures before pushing up into urdhva dhanurasana. If it helps, then you know tight hip flexors were at least part of what was restricting you.