Two reasons, like it or not for the 30 round mag issue.
1. The rifles were not designed to handle them, they were designed to handle 20 round mags. That has as much to do with the part and pieces of the rifles as it does the ergonomics....Try to get down and dirty with a 30 rounder, and you will figure it out very quick. They just get in the way.
2. You add 10-12% more weight to the rifle. Harder to carry for 20-35 miles, harder to shoulder and fire with accuracy, harder to deal with the logistics of supply.
The FAL 30 rounders were select issue items. I am not aware of issue HK or CETME mags, and the M-14 was NEVER issued with anything but 10 and 20 round mags. As a matter of fact, the 25 round mags that were availible as Gov. Mil. Spec were rejected due to the ergonomics and the reliability.
My best advise:
If you want 30 round mags, buy an AR or an AK. If you want an LMG, buy one and lots of links. If you are fortunate enough to own a fine Battle Rifle in 7.62.51 NATO, stick with the issue mags. They go bang every time.