I'm not really sure why Bethesda didn't just make most of the NPCs marry-able. I can understand why Jarls and a few other NPCs might be off-limits, but I really don't see any logic behind making
so many NPCs un-marriageable. I'm inclined to think it may just be because Bethesda rushed the game out by 11/11/11 and some thing just didn't get implemented they way they were planned.