It may be no surprise to hear this from me, but I put a lot of the blame on the current confusion on the previous administration.
Since they were not keeping track of what states were doing, when they heard that states weren't vaccinating fast enough to meet the Fed's announced goals, they... panicked. They told states to move on to 65+ when the states hadn't even scratched the pool of 1a recipients. And so, anyone 65+ jumped ahead of the rest of the 1a population and those targeted 1b populations. This opened up a pool that the number of vaccines available couldn't handle. All because they weren't keeping track.
And when this led to shortages in the states, the head of HHS said the reserve will be released... except it was already released. The initial weekly doses being shipped out was high because they were coming from the stockpile created before approval. At some point, that stockpile was going to be depleted and then the doses would only be as much as could be produced week by week.
But, the previous administration wasn't keeping track of that at all.