Main reason? Hmm?
Debatable, but undoubtedly a large reason. Much of the Arab world is scared of Muslim extremism and terrorism and they also really don't want to make millions of Palastinians directly their problem, thus the borders locked for decades. Not that that that justifies Isarael's behaviour in general.
This is certainly the case for Egypt and Jordan (the former being the middle east's most populous nation). Sisi doesn't want to deal with infusing Palestinians into his population after having gained power by overthrowing a Muslim Brotherhood guy. Jordan is already about 50% Palestinian, so the King won't want to risk destabilizing himself by adding more.