I think it's the upfront cost, which is still a problem.

The only reason I ever got my Mirena IUD was that it was the first month Obamacare kicked in for mandatory birth control coverage and my insurance covered it. Before then, I wasn't willing to entertain the idea of an $800+ option when I was doing okay with other things (mostly nuvaring). Mirena was by far one of the best forms of birth control I had ever used though :)

There are definitely problems with the procedure and the IUDs themselves for plenty of women, too, let's not ignore that. But it is hard to screw up and it is a pretty easy method. I hope that better coverage leads to more IUDs and less unwanted pregnancies :)