The disadvantage of a bullpup (ambidextrous operation, for example when firing around corners) can be neutralised mechanically, but its advantages are still mostly diminished by

* forward mag weapons getting folding buttstocks (can be even shorter than bullpup if folded)
* overall moderate required barrel lengths in comparison to the early rifles
* shorter sight line for (backup) iron sights
* greater difficulty of reloading under stress

The difference is simply not large enough to require a general conversion to bullpups.