Hamas terrorizes Palestinians, threatening those who dissent with cutoffs from basic amenities and even certain death. All of the aforementioned militia have good reason to distrust PA, because PA is the recognized representative of the Palestinian people by every single country in the world. No country gives a shit about Hamas. When aid is delivered to WB or Gaza, it's delivered in the name of the PA, even if they have lost control over Gaza for so many years.
And why does Hamas oppose PA? Because their ideal government is one with roots in the Muslim Brotherhood, which is a designated terrorist organization in the West as well as every surrounding country in the Middle East.
One could argue that Hamas is the rightful representative of the Palestinian people. But is it really? Elections held in Palestine are often a sham affair, with threats and coercion abound. But even if they won with a resounding majority, the fact that Palestinians en masse chose to elect an organization that cuts their water supply to make rockets from pipes says a lot more about the kind of people Palestinians are, and why they shouldn't be supported too much (something which every Arab neighbour of theirs has figured out pretty much).
