Could be wrong, but I'm pretty sure that "bullpup" refers not to barrel and weight specs, but to the placement of the trigger group well forward of the action, and the action located well back into a short stock.
I think the arrangement is designed to improve on the carbine concept by not limiting them to just a shortened barrel and stock. The reconfigure allows one to get the much shorter overall carbine length, while still maximizing barrel length.
As for why there are not more weapons of this configuration, I can't say with authority, but I'd hazard the following guess;
Bullpups do in fact, as Hang pointed out, have shorter carbine-length @18" barrels, whereas battle rifles typically have 22"-24"+ barrels for improved range and accuracy. Bullpups therefore have a place wherever a shorter, carbine style weapon would be appropriate, i.e...some special forces applications, some airborne applications, tank crews, etc... OTOH, don't forget how powerfully tradition influences arming choices for miltary services.
Didn't a large military just equip with bullpups, somewhere? The ANZACS perhaps? I don't recall precisely...anybody know for sure?