From John on Patreon:
“You’ve covered your fair share of strange conversion of firearms. Have you ever come across a bullpup conversion that was actually useful, and not a kludge?”
That’s basically the whole answer.
Okay, to elaborate a bit, it’s because making a bullpup use blue from either shoulder requires specific accommodation of the empty case ejection, and converting a standard rifle to bullpup configuration basically guarantees it will only be usable right-handed. Also, designing a good bullpup trigger requires specific intent in the design stage, and conversion triggers are always kludges. And lastly, the sights often end up mounted to some wobbly plastic extension.