I think it's a combination of two things: first, ineptitude on the part of Samsung Mobile (which is, after all, not a software company); and second, Verizon's rigorous testing. The first is a bad thing, and the second is a good thing. But both mean delays.
There is plenty of evidence that Samsung has been trying. They've delivered multiple builds of Froyo to Verizon; unfortunately, it has taken them forever to do so, and the builds have been too buggy for Verizon to release. (This would also explain why Samsung is so tight-lipped about what's going on: they can't exactly admit that they keep screwing it up.)
I wish Samsung were better at this, but they're not. And given their apparent quality problems, I'm glad Verizon is testing the hell out of it.