Cloaks are only practical on ships that keep their power use to a strict minimum: stripped-down, no-frills warships. No holodecks, no science labs, no arboretums, not a single luxury. So a ship like the Enterprise or the Titan couldn't practically use a cloak anyway. If you ask me, that's the real reason the Federation agreed not to develop cloaking tech.
Which is why the only federation ship that has it is a Defiant class. Makes sense.
(I'm ignoring the novel Crossover, which had Scotty bootstrap a cloaking device onto a Constitution class ship)
I always figured Klingons got the cloak as part of their alliance with the Romulans, and couldn't honorably refuse the technology without offending their ally.