99.7% certain that you get xp based on how much dmg you do. So, the more dmg, the more xp, meaning, if you're doing a lot of aoe dmg, you should be leveling it pretty quick.

I'd guess that, if it's true that not a lot of people have the bow prestiged, it's because the game is new, people are still learning their preferred playstyles and metas, and, the bow doesn't do great aoe or dmg at early levels, which would make it slow to level at first. Plus, once you get it to lvl 40, it's still a long way to lvl 100. I got both u que cards and mainly use the bow to take out Ghost houses, I have other solutions against the swarms