Many reasons for preferring the FAL over the G-3:
1: The impression that the FAL was better because of favorable reviews when tested as the T44 agains the excellent M-14.
2: Less stamped parts on the FAL. Largely a falacy, many Americans are of the opinion that stamped parts are inferior.
3: Fluted chambers. Your brass looks like crap and, although you can reload it, most feel it's not a good idea.
4: Vigorous ejection. Unless you have a port buffer, the brass gets the crap beaten out of it and is sent out almost as fast as the bullet!
5: Availability. For many years and even today, the FAL in it's many incarnations is easier to get hold of.
6: Cheaper magazines and accessories.
7: Roller-Lock discrimination. Most (including myself) like a good, sturdy, simple gas system that's easier to clean and gives us more peace of mind.
8: Economics. H&K marked their gun in the US early on at very reasonable prices. As the world market made the Dollar and Mark exchange less-favorable, we got a 'cheap' gun that ended up very expensive. This transleted to accessories, spare parts, and magazines as well.
The biggest complaint I had was that, when I wanted a scope mount, they wanted $200 for it. The magazines were $40 and tools and parts were outrageous. I didn't mind the brass dentning or stamped parts. I did get perterbed when I tried to shoot 600 Yard slow-fire at a DCM match and found there was no 600 meter setting on the drum site. Apart from that, my gun (a Greek SAR-8) was quite well made, accurate, and ate anything I fed it.
That being said, I now own an AR-10 and an FAL... Hmmmmm.