The Daleks were used eight times (not counting the Five Doctors cameo) in twenty years. They went five years between appearances during Tom Baker's reign, Baker only facing them twice, and the three eighties Doctors only facing them once each. It's really nothing new to rest them periodically.
Wasn't one of the reasons the Daleks were so scarce back then becasue Terry Nation was being a bitch? It's possible if he were more cooperative, we'd have seen them more often.