Because an HK SBR killed Osama Binladen. That ought to be enough for anybody.
LOL!...but seriously, SBR AR's are popular because like any AR the parts configuration is almost limitless. You can also tune an SBR AR to just about any suppressor and/or ammo. Using very decent parts you can probably build two SBR AR's for the price of one bullpup. I own a 10.5", 14.5", 16", 18", and 20" 5.56 AR...along with a pair of AR10 .308's. I also own a Tavor, AUG, and RFB. I like them all, and they all have a place. I think there's a clear reason many special forces units use the SBR for missions where long distance shots are highly unlikely. At a full 100 yards my 10.5 AR with a red dot shoots about as good as my two 5.56 bullpups with red dots. The military and special forces units are extremely familiar with the AR platform, and for closer quarters, the SBR AR's do quite well. The real beauty of the bullpup is the ability to bridge the gap more effectively between CQB and longer distance shooting. After a 100 yards my 10.5 AR degrades quickly in accuracy and terminal ballistics. Military guys aren't usually going it alone. They work in units of varying sizes with weapons selection that cover the bases for the given mission. My 10.5 AR was my most recent AR build, and I was surprised at how well it performed. It is basically the same length as my Tavor and AUG when the stock is fully collapsed. No one gun does it all for all shooters or all conditions.