Why no Su-27s or MiG-29S over Afghan'?

Okay, slight correction to the title – the Soviets did use the MiG-29 over Afghanistan but only against one or two Afghan’ Su-22 “Fitters” whose pilots disagreed with Soviet involvement.
Why, when the Pakistani Air Force started to engage Soviet MiG-23s/Su-25s and various Afghan fighters loyal to their government that flew over the Pakistani border to bomb “refugee” camps – did the Soviets not decide to counter Pakistani F-16s with Su-27 or MiG-29 escorts??? Certainly would have made the Pakistani pilots think twice ’bout charging into battle!!! Plus – if the government had removed that stupid rule of Soviet pilots not being allowed to engage hostile aircraft over the Pakistani border – the Soviet “Fulcrum” and “Flanker” pilots could have wiped that smug grin from the face of the PAF, I doubt the PAF would be as quick to boast about it then!:p
I know about the political…ehhh…situation involved in any such encounter, but still it would have been interesting.
I also know that both these fighters were only just entering service, so was this a reason for not commiting???