You know, that's always puzzled me-- I recall reading in the 1980's
that they thought Air to Air UCAV's might actually beat CAS versions,
due to the problems of picking targets out of the ground, IFF, etc.
On the other hand, Air to Air could be done by selecting a section of
air and declaring anything in it hostile, giving the UCAV's free
range.

What changed? I'm assuming that at least part of it is the fact
that unlike the 1980's we're not talking all out war, so it's very
likely that even "war" zones may have to worry about civilian air
traffic and nobody wants a repeat of the Iranian shoot down (although
that wasn't a UCAV).

I think it would depend on what kind of UCAV. I've still not heard
definitively whether they'd be controlled by a pilot on the ground or
if they'd be 100% autonomous or some combination. It would also
depend on the kind of air to air. Is it just medium to long range
shots or might it include dogfighting? I think the questions are can
it dogfight autonomously and can it be trusted not to down a friendly
aircraft. Also if remotely piloted would the guy on the ground be
able to see as well as a guy in the cockpit in a dogfight and will the
communicatons link stay up. Things like that.