They only have so much vaccine supply. They are opening it up in tranches to keep using the doses They have and not overwhelm the system too.
At this time I don't think there is anything more to it than that. Pfizer is still being used for 2nd doses although that may change soon. Supply of Moderna is low and I think will be less than expected in May.
If they only have 1 million 1st doses then they only want 1 million to book. On the other hand, they don't want to give out 0 doses because everyone who wants it has already booked.
There are still a lot of over 40s still to be vaccinated (I am one of them). I'll take whatever there is available.