It takes them months to make the vaccine. The just can't open a faucet and have more vaccine come out.
Basically, the public health officials, hospitals etc... estimate how much vaccine they will need about a year in advance, and they order that much. Taxpayers and hospitals can't be expected to pay for way more vaccine than needed year after year, only to throw millions of dollars away 'just in case'.
If there is a greater demand than expected for the vaccine, you've got a shortage. This is the first time I can remember where there is a nationwide shortage - it's not like the screw it up every single year.