Because many of those who are vulnerable don’t know they are. Including many who have been vaccinated once.
I know someone in hospital now - fit, healthy, well under 40. He’s had a very difficult few weeks. He also, unwittingly, passed it on to his immediate family; thankfully none of them needed hospital treatment. He has no idea how he caught it - early May, working at home, keeping to the restrictions.
Whilst one example doesn’t prove a trend (far from it) it is reasonable to assume that had all restrictions been removed he would have been back at work, on the train, in the pub, etc etc., and that he would have unwittingly passed it on to a great many more people. Some of whom would also have been vulnerable without realising it and ended up in hospital. And so on. Had he been fully vaccinated I doubt he would have caught it, and certainly not ended up in intensive care.
Until we get sufficient people vaccinated to tigger ‘herd immunity’, or something like it, there is the risk that sufficient numbers of them could get ill enough to be in hospital that causes a stretch in the health service. As it happens I think we are pretty close to getting herd immunity, but not just yet, as events in Bolton have shown. But I do think that by 21 June we will have enough to do what is indicate din the roadmap, subject to the work that has yet to be published on a few matters.