Blessed Strikes gives you this option that doesn't require a bonus action or concentration:

Divine Strike. Once on each of your turns when you hit a creature with an attack with a weapon, you can cause the target to take an extra 1d8 Necrotic or Radiant (your choice) damage.

This damage increases to 2d8 at level 14.

What is it about the ranger that makes hunter's mark overpowered compared to a cleric with divine strike?

The ranger has limited uses (Wis modifier spell slots), must use a bonus action, AND had to concentrate. Meanwhile, a cleric just deals the extra damage without having to do anything.

Looks like thoughts lost to prayers...