We didn't need to wear masks early on, because we shouldn't have been going out except for essential shopping or essential work etc. Far fewer people out and about so pretty pointless wearing a mask if the 2 metre social distancing was being complied with.
From Monday, we're in a completely different place, with public transport back open to all, people going to work, people shopping, people going to zoos, etc. Far more people out and about increases the risks and makes it harder to maintain the 2m social distancing, so makes sense to encourage people to wear masks.
So not really comparing like for like is it? Circumstances have changed, so rightly advice needs to change accordingly.