why is it the case?
The whole "Palestinian" sub-group of Arabs is highly artificial... it's a combination of locals who have roots in the area before 1947 and newer arrivals attracted by employment opportunities created by Israel (IIRC Yassir Arafat was actually from Egypt). The different factions don't get along by themselves and are only united by hatred of Israel.
It's also divided in terms of religion (the Christian element used to be more prominent, I remember when Hanan Ashrawi was one of the most articulate voices of Palestine) but they've been pushed out by the Islamic revival that started in the 1990s.
Within the group "Palestinians" you have some of the more educated Arabs (working in other countries and office environements that locals often coulnd't do0 and some of the least educated and least adapted to the modern world.
Other Arabs look on them as losers (vis a vis Israel) and a threat in terms of some of them being better educated and understanding concepts like political opposition that most Arab countries don't get.
There's also the land question... Islam (more than other religions) claims land and the idea is that once a place is muslim majority it 'belongs' to the islamic world... they're still not over Spain and Israel is a very fresh wound that dishonors (in muslim minds) the entire muslim world.
There's more, but that will do for starters.