>>64001188I think they just have less cultural cachet than other battle rifles.
The FAL has the whole "right arm of the free world" thing, the Rhodesia thing, and was the service rifle of most of the English-speaking world (outside the US obviously) so it's got a lot of fans.
The M14 has the Garand heritage, the traditional looks, legality in ban states, and saw half a century or more of US service. (Albeit very limited for most of that.)
The SCAR and AR-10 derivatives have modern features/manuals of arms, are in current service, and have been in a billion vidya etc.
The G3 is way behind. Outside of the general HK memes and the occasional movie or vidya appearance it's comparatively obscure.
On a more practical note though, I think the G3 loses some popularity because of the roller-delayed system as well. It results in heavier recoil than its competitors and it's also known for tearing up brass, I'm not sure how many people actually reload .308 but I often see that cited as a reason people don't like them.