Star Trek: The Next Generation/Cloaking devices
Why is it that both Klingon and Romulan ships cannot fire their weapons while they're cloaked?
Hi David, thanks for the question! I don't think the limitation of cloaking devices is ever explained in the TV shows or movies, unfortunately. It is possible that an explanation was given in an obscure episode that I'm not familiar with, but it definitely wasn't in "Next Generation".
But we can speculate:
From a "Star Trek" universe point of view, it might be that the cloaking device consumes so much power that not enough is left for the use of weapons. Or it could be that the cloaking device depends upon a completely contained field being maintained around the ship, and a beam or projectile weapon would disrupt the field.
From a "real world" point of view, an enemy ship that is completely invisible while still capable of attacking and destroying the good guys was probably seen as too powerful an advantage to allow, given that the good guys didn't have cloaking ships themselves (which is the subject of another discussion entirely).
That being said, one of the plot points in "Star Trek VI: The Undiscovered Country" was a prototype Klingon bird of prey that could fire when cloaked, but we never see or hear of this technology again after that.
Anyway, hope that helps a little. Thanks again for the question!