Are transphasic warheads that far ahead of the technology curve?
They come from an alternate timeline only a couple of decades in the future. I wouldn't be surprised if Starfleet R&D types had been working on the technology already, maybe even knowing the requirements while not being able to fulfill them, only to be surprised by the sudden introduction of the completed technology.
If that's the case with the Federation, I suspect the other major powers--including the technologically-advanced Romulans--wouldn't be far behind. We know that Klingon military technology was capable of producing transphasic torpedoes when given the technology by Starfleet. Other militaries would presumably be capable of the same.
As for why Starfleet particularly hasn't made wider use of transphasic torpedoes, I suggest that it might be because, apart from their potentially arms-race-sparking destructive power, they would be too powerful. If Starfleet can destroy ships with only one easy shot, that makes it very difficult for Starfleet to de-escalate confrontations. Having transphasic torpedoes as part of a ship's munitions could make things go bad more easily.
Maybe, but we just don't know. To my knowledge, the only thing ever shot with a transphasic torpedo was a Borg ship. We have no idea what effect, if any, it has on anything else. Might be awesome, might not be worth it.
Perhaps contemporary science could only partially implement the technology, it being from the future and all. Look at how Slipstream has been somewhat... Experimental ever since it's been adopted (not from the future, obviously, but the advanced science can be seen as analogous).
Weren't there also Transphasic Shields, as well? My memory fails me at the moment.
Meta fictionally, though, the simple answer is that Transphasic torpedoes aren't interesting. TrekLit isn't about being Military SF, so the writers don't concern themselves overmuch about things like an upgraded torpedo, unless a good story opportunity can be made out of it. Compare this to, say, Slipstream, which was a game changer, and a very welcome and necessary one for continued stories, as conventional warp drive had been written into a figurative corner as simply being too slow for the kind of stories which were coming out, especially with the new trends toward exploration of the Beta and Delta quadrants, which otherwise would have required long haul, almost generational ships to accomplish with traditional drives. Slipstream nearly avoids those logistical nightmares while helping to move the universe forward a bit more. Transphasics merely open another arms race that would only end in parity again, once everyone figured it out for themselves.
