There hadn't been a Dollar coin minted for 50 years, they probably figured that they could make more money selling it single. This way they could charge their surcharge on tne mint sets and the Ike if the collector wanted it.
Well for the 1971 sets I believe they came out before the Ike dollar was ready for release for circulation. As for the 1972 they probably didn't allow enough lead time to to permit the preparation of the heat sealing die. (although you would think that they would have thought of that back in early 1971). It may also simply have been that they had to wait until their current contract with the form that made up the mint sets expired before they could make a change in the set content and layout.
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