In a word: looks.
The first "shorty" ARs (the XM177s) had 10" barrels (and this "set" the length of the carbine handguards and gas system), but they weren't very reliable, so the barrel length was extended to 11.5". Still, 4.5" blast and flash was so high that a combination flash suppressor/sound "moderator" was designed and installed on the later XM177E2 models. Eventually, people realized that if the effective length of the barrel was going to be around 15", then you might as well make it all barrel, and get the benefits of all that powder burning.
Amazingly, there was very little thought given to the terminal ballistic consequences of a short barrel at the time. It was just assumed that they would work. We know much better these days, but some folks still choose looks over performance, and think the long flash suppressor on the short barrel looks cool. Whatever.
-Troy