The military rarely changes their requirements unless there is some outside reason for them to do so.
In the late 50s/early 60s, plastic technology wasn't very advanced (most "plastic" was Bakelite), and cast aluminum alloy followers were the best option for followers. The Mil-Specs for the 20-round mag were set in the early 60s, and included the alloy follower.
By the late 60s, when the 30-round mags were designed, plastic technology had advanced to the point where plastic followers were both a viable replacement and less expensive. So, the spec for the 30s was a plastic follower.
Still, the spec for the 20s continued to require alloy followers, because they didn't want to change the specs. The last contract for USGI 20-round AR mags was in 1971. 20's made after that were made for civilian sales, and plastic followers were used, as there was no requirement to follow the old Mil-Specs for civilian equipment.
Another unrelated example: until about 2 years ago, DoD was still buying carbon-based batteries instead of the alkalines that the rest of us have been using for 20+ years. Why? Because that was the Mil-Spec, which hadn't been updated since the 60s. When the contract was up for renewal in 2000, none of the battery companies would bid on the contract, as they no longer had the equipment or supplies to make the old, obsolete batteries. So, the spec *finally* was updated to require alkaline battery technology. Welcome to 1975!