While strictly a regent of IHH may not have to be a priest of Haelyn its a fairly safe bet, so Haelynites should be able to bring people back if need be.
I'd say that resurrection is rare because:
1. Leaving heaven - if gods invites someone in, it is very rude to drag them back out again
2. Who says you forget heaven anyway? Frankly leaving people dead lets the priests perpetuate all the cherished rubbish about heaven that they think is important, why contaminate faith with reason and risk schism?
3. Inheritance, rival claims to old property/spouses, etc, etc as Matt said, being dead is serious business - and coming back causes all sorts of family rows.
4. The impact on life being sacred - if dying is temporary why is murder worse than assault?
5. Tearing holes in the veil to bring a spirit back from the dead can have unfortunate side effects in BR, similarly doing so casually invites demons to encourage those unprepared to attempt the feat risking unleashing demons into the world.
6. Losing the bloodline - I see the bloodline as being a fragment of the soul, the source of creativity, inspiration, etc, etc - if that spark is lost what has truly returned? Have you brought back a man or merely the ghost of one gone?
7. Dying is partly the DMs way of saying that you've been dim, why encourage Bjorn to drop a thousand tons of masonry on people to get the point across that minion X is gone? It will only increase the casualty rate!
Etc, etc.