Because it'd be pointless.
Omicron has an R value between 3 and 5. Even a March 2020 lockdown isn't going to halt that, particularly now when people will mix at Christmas whatever the government says (and I don't blame them, honestly). And even if restrictions did slow it a little (and don't forget, there would still be plenty of settings where it would continue its exponential spread), as soon as thing reopened it would go bonkers again.
In the past lockdowns have bought us time to roll out vaccines, but that's been done, now. Most people will have some level of immunity, making illness milder. The knotty point is how well the NHS will cope with the (possible) huge numbers of people who will need help - but even at that point, lockdown won't do much as the crisis will be here.
I think there'll probably be some restrictions (nightclubs closing, wfh continuing, table service in pubs etc) but even those won't be without great cost to livelihoods and MH. But the peak of Omicron looks as if it might arrive very quickly indeed, and hopefully pass as quickly too. Who knows? By early February this we could have a situation where, although there will be a rise in deaths and hospitalisations, millions of people will be triple or quadruple jabbed and had covid at least once (if I catch it it'll be a third time!). It'd take one fucker of a future variant to break through those defences, and Chris Whitty has said that another like Omicron would be very unlikely.
But by God we have a mountain to climb first - not one that lockdown would help in any meaningful way, however.