Because I sure can't see why I would ever, under any circumstances, want to use one instead of some of the other, vastly more useful enhancements.
First of all, we have two weapon slots. In most situations, this significantly reduces the need for reloading. Some loadouts, like assault rifle + sniper rifle, may occasionally not be able to properly employ one of the weapons (you get swarmed by melee enemies, for example), but as long as the player initiates combat on their terms, that shouldn't be a common problem.
Secondly, Bullet Strap Belt exists, and it brings the AP cost of reloading from 15 down to 4. That's almost nothing. There are good competitors for this equipment slot, though: Doctor's Pouch is excellent, and you may also want to use either Utility Belt or Trapper's Belt, depending on you build. It's not a no-brainer choice, but for burst-fire builds Bullet Strap Belt does make Extended Magazine essentially pointless.
And thirdly, the actual effect of Extended Magazine is pretty weak, only increasing maximum capacity by 6 or 7 (depending on whether the increase is 20% or 25%) at best, if base capacity is 30 rounds. One more burst per reload, basically. Really underwhelming, considering the previous two points.
So, in my humble opinion, both Extended Magazine and Extended Rifle Magazine are in need of a significant buff.