Reference why the IDF doesn't like the FN FAL, basically, the guys who had to use it thought it was too heavy, to clumsy, and didn't like the way it performed in the desert conditions they often had to operate in. Even years after it's adoption, some guys were still hanging on to the 98k Mausers that Israel had bought from the Czechs in the late '40s. Remember, Israel had adopted one of the earliest versions of the FAL, as it was one of the only semi autos available on the market at the time. The few other types that were out there on the market were even worse (does Swedish Lyungman ring a bell?). After they captured a bunch of SKS's and AK's from the Egyptians, they got a taste of rifles that could funtion reliably in sand, and y'all know the rest. The guys on the line in the desert even preferred SKS's to the FN's.
Don't get me wrong, the FN is a good rifle in nondesert areas. It just doesn't work well in the sand. Look at all the Brits had to do to get even halfway decent desert performance out of theirs. Special sand grooves, removing the bolt holdopen so sand wouldn't blow in while the action was open, etc,etc.