There is a good deal of armed forced using bullpups so I think your failure comment is a tad overblown.
As for the number of competition shooters who use the AR-15 platform I think that has more to do with numbers of rifles actually out there and the fact the platform has been so developed by so many people for so long than superiority of platform.
I believe that if the same amount of money and development time had gone into every bullpup and if they were adopted by the US and other NATO countries you would likely see a lot more bullpups in competition.
SO I don't think its a platform or layout failure more than a simple numbers game.
I'll briefly discuss the lack of merit in a bullpup rifle.
Not ambidextrous
difficult loading/reloading
fixed length of pull
prone shooting can be difficult due to rear placement of magazine
Some have ridiculous mechanical offset
Clearing stoppages(type 3 or worse) is problematic
Lack of aftermarket support
You are however correct, that due to the insane amount of aftermarket and OEM goodies available for the AR system, it tends to be more prevalent. However, accessories alone do not make a system popular or successful. The system is proven, both in form and function. Its reliable, and its ergonomic. Not to mention its ambidextrous from the box and can be made 100% ambi with accessories.
As for militaries using bullpup rifles. I can think of two, and a half. The Austrians and the Australians, and the Israeli's but they make up the half as not all of their forces are running Tavors. Then again, not all of Australia'a forces are running AUG's?
There's no doubt some bullpup rifles are reliable and fairly ergonomic. The problem lies in operation. As mentioned, they present problems when dealing with stoppages and they're not ambi or LOP adjustable. Both are major issues when discussing issued service rifles. From that standpoint, bullpups as a whole are failures.
TDC