>>198797709Bloodseeker's major disadvantage is that he isn't actually very strong in a one-on-one confrontation. His damage output is really quite terrible by default and all he has outside of Rupture is a silence that won't work on any carry with a threatening autoattack. Bloodseeker going mid is important because he really needs that level advantage and lucrative farm to do some damage.
Bloodseeker can survive in the jungle and farm, yes, but he can't do it terribly quickly or efficiently, which means that you can easily lose out on his ganking potential. From there on out, you're just a sub-par carry. He's also totally defenseless if anyone happens by the jungle while you're in there. He's an incredibly easy hero to gank whenever Thirst isn't active, and for that reason needs the safety of the lane.
There's a million reasons to go mid as Bloodseeker, and very few to jungle. If you want to jungle and gank, you might as well pick another jungler.