Because the UK govt is deeply worried that it's lost its authority and anything they say will be ignored.
I seriously doubt that - at least in the sense I think you mean. If they say "pubs must close" they will close. The government has all the necessary machinery to enforce that decree.
The real issue is that this dilemma is a microcosm of the larger problem, namely: how do you balance getting the economy back without filling the hospitals with coronavirus patients. If the government gets the balance wrong, it will end up losing its authority in the next general election.
So first they put up a bit of kite flying through sources hoping to get people agreeing to the schools choice.
Much as I like a pint in the pub, getting the schools up and running must be a priority. No reasonable person would agree that pubs should have priority over schools.