There is alread a pretty reasonable canon explanation for it: The ENT-era Romulan cloaking devices as well as the Romulan devices operate on "invisibility shields" - They make stuff invisible, but it's still there. They can make starships look like empty space ("invisible"), or like other starships (the ENT Romulan three-parter), but if you hit an invisible mine, it's still there.
The TOS-era cloaking device operates on a comnpletely different physical level though: It phase shifts the starship. It's not just "invisible", it's physically not there, but in another realm, so that it even can "cross" through solid matter (TNG: Pegasus, and the "Geordi and Ro are dead"-episode).
The first one (invisibility shield) is something very common for TOS era Spock. The latter one (phase shifted cloaking) is something completely out of the realm of Spock's (and Federations) knowledge.