Who said they were phased out by TIE interceptors?
ROTJ and common sense say so.
Why does the B-52H, last built in the 1960s, still make up nearly half of the US heavy bomber force? Why does USAF still fly versions of the F-16, initially developed in the 1970s, despite the development of more advanced airframes such as the F-15, F/A-18, and F-22? Can’t seem to figure out what common sense you speak of.
Because Soviet Union collapsed (effectively in mid 80s). Besides, B-52 was militarily useless since the introduction of ICBM. It has been used for nothing but bombing farmers and 3rd world countries’ cities since Vietnam War.
If you want a good analogy, take WW2 (ANH-ROTJ) and post WW2 cold war (post ROTJ) as an example. New models had been developed and replaced the existing ones constantly.