I once read elsewhere that the move away from older fashions was a budgetary thing. Apparently the producers just didn't want to incur the extra expense of period clothing (or the network had really clamped down on their budget).
Off-the-rack stuff available in all sizes was no doubt cheaper than vintage wardrobes, which would likely also need more work in the way of alterations.
In memory of lovely Erin Moran 1960-2017 ~ Missing you
"For you are beautiful ~ And I have loved you dearly ~ More dearly than the spoken word can tell..."
"What's the word?" (Paul Martin) ~~ "I don't want money for nothing." (Timmy Martin) -- Lassie ROCKS! WORD UP
"It's just a dugout that my dad built... In case the reds decide to push the button down..."