Because they gave control back to ministers to vote on any restrictions - he would have to rely on Labour support to get anything more stringent through. The obvious answer was stricter measures during the last 2 weeks whilst the vaccination programme ramped up - but no-one was brave enough/politically foolish enough to do that. (I feel all the leaks to the press about covid violations have a very antilockdown agenda attached to them - to encourage non-compliamce by general public)
Really the spread is totally baked in now, and will only be intensified by household mixing at Christmas. Given the lag between infection and hospital admission we don't know what the effects of Omicron are yet. Given big rise only happened over this last week (and likely to continue and accelerate) We won't see the affect on hospital admissions for another week (just in time for Christmas) and (because Covid keeps people in ICU for a long time) any increase in death rates long after that.
This gamble on boosters is politically motivated.
They should never have had the rhetoric around freedom day, we should have had consistent messaging about masks and their vital role in public health. Using simple measures might have kept the daily high covid rates much lower, so that when Omicron hit, we had some wiggle room.
The vaccine programme has been amazing - but until the pandemic is over- it was never going to be enough.
Honestly what ever they decide to do is going to be almost impossible to implement.
Even just 2 household mixing at Christmas will encourage more spread - given the higher tramsmissability of this virus and the fact that most spread is between household contacts.
All we can hope is that the booster programme holds up, and that remaining people get a milder, non hospitalised case. But we just don't know yet. And even small numbers on a population level can cause huge health resourcing problems.