Su-17M3 and M4s were not old, they were produced up until 1990 I think. MiG-23MLD had plenty of service life left as well.
Military drawdown was obviously the biggest part of it, but I think anti-single engine thinking was there as well. I mean on a shoestring budget some single engine types probably would have been cheaper to operate than an entirely big twin-engine force.
EDIT: Not sure if this source is legit or not but it quotes Deynekin, Commander of VVS:
"Как указывал Главком ВВС П. С. Дейнекин, «переход к использованию самолётов, оснащённых двумя — четырьмя двигателями, существенно повысит безопасность полётов»."
Pretty explicit decision to discard single engine types, allegedly due to safety.