Maybe it isn't but it supposedly lasts forever. Anything that lasts forever will get tedious with time. Let's assume heaven is like the Islamic version with 72 virgins and all kinds of pleasures. That would be fun for a while... After a thousand years it would be less fun. After 100000 years it would probably be unbearable. When a million years have passed most people would want to die... But they can't since it lasts forever and they know this. The only way it could be bearable is if it's some existence completely different to this one.
Pretty sure most religion's ideas of the afterlife don't have you possess the same sort of suffering consciousness. I imagined it being sort of like a passive euphoria all the time.
If it wasn't like that, it'd have mostly uptight people whose standards aren't worth the effort of living up to, a strong degree of conformity, while hell has variety. With enough time, Hell sounds like the better choice from the eventual jading that'd follow perpetual suffering, and following that would be the potential to meet a large span of people who are more honest with themselves than the pretentiousness of heaven.
Of course this is all speculation on a fake concept, as the whole Heaven/Hell concept is blatant reward/punishment of a slave religion. It reeks of human wish fulfillment and control, making it the less likely truth from how blatantly it smells of the human condition.
Maybe but what if there's no escape? It's not like death is an option any longer. This notion reminds me of of the Hindu and Buddhist concept of reincarnation. The cycle of birth, life, death and rebirth is supposed to go on until the human becomes enlightened, and can check out of it. Then what remains is a state of neverending bliss. Some versions of heaven or the afterlife is like a continuation of the physical existence though. That's the kind that could tend to become dreadfully boring with time.