Have you ever had a bullpup with a good trigger on it? I am far (far) from an expert on pullpups but I have never found one with a nice trigger. Also, they are more awkward to load. Nothing would be able to change that.
Bullpup shotguns are also more susceptible to short stroking the action.
You can fix the trigger, but I think the other two issues rule out the bullpup for me.
Personally I don't mind the trigger on my AUG and as such I haven't bought the parts to clean it up (I'm lazy), but there's multiple things out there to help lighten the trigger pull for it. So that tells me it is completely possible to fix up the trigger and it's simply a manufacturers' issue and not an insurmountable problem with the general bullpup design (the trigger mods I've seen for the AUG drop it down to ~5lb).
As far as being awkward to reload, well pretty much any gun is going to be awkard to reload when compared to the AR design. That's one of the big things about the AR platform. Though one can always train to help make it not as awkward. And some people might even like the fact that the mags don't just free-fall out of the mag well for various reasons.
For bullpup shotguns, the only one I even really know about is the KelTec one which is a pump. Though the fact that shotguns may or may not have this problem shouldn't affect one's opinion when looking for a rifle or at non-semi bullpup shotguns.
The main thing about the bullpup design is that it gives you a shorter overall gun while retaining a longer barrel and how it changes up weight/recoil distribution. The shorter design but longer barrel make it better when one needs/wants to be able to switch between CQC and further ranges. It can also be nice for a hunter as they need less space to maneuver their gun (not a big deal if you have a wide-open area, but could be helpful if hunting with a friend/family in the same blind or depending on the foilage of your hunting spot). And by moving the bolt and mag further back the weight/recoil is closer to one's body. This helps lower fatigue over extended periods of time and helps one to control the recoil better for follow-up shots.
Not that there aren't potential issues with the bullpup. The reloading can be awkward and require extra training compared to an AR (but again, pretty much any gun is going to be a bit awkward to reload when compared to the ease of the AR-style). It's not really a design that would do well for bayonet use (though I would say the new collapsible-stock AR design isn't that well suited for it either). The gun is more complicated which can make it harder to work on and more expensive in general. Historically they don't have the best of trigger-pulls (something that can be fixed but might require more of an aftermarket solution). And historically they haven't been very friendly when one needs to switch between left/right hand use in the field (though I am seeing more and more truely ambidextrious designs coming out so this is going to now depend more on the specific gun and has less to do with the actual bullpup design).
Personally the advantage of the shorter length/longer barrel and the redistributed weight/recoil outweigh the reload speed and relative simplicity of the AR. To each their own, but I just don't want to see people simply going with the AR-style out of ignorance/tradition without truely understanding some of the pros/cons of the bullpup; especially with some of the advances in design lately in regards to the trigger and ambidextrious use.
EDIT: Oh and I would say part of the problem of the bullpup vs AR is that the AR is basically "America's Rifle" while the bullpup simply isn't. This has led to most Americans simply getting what they know (which leads to more development money for the AR to fix issues along with a bigger reputation), while bullpups are rather obscure to most people and simply haven't had the R&D funds poured into it to fix some of the issues until recently (hence the trigger and ambidextrious issues that have often been associated with bullpups).
Pax...
