One of the problems is that people struggle to deal with the concept of risk, when it comes to very rare events. And also people's complacency increases with time. So bad events come as a real shock, eg home destroyed by floods or being diagnosed with a critical illness, even though maybe you've known for years you lived in an area prone to floods or have had lifestyle factors (obesity/smoking/drinking) that predisposed you to higher risk. The longer people live with risk, the less they worry about it, so its a shock when something happens. When there's been a terrorist attack, people are nervous in train stations and stadiums and taking the tube for a few weeks, but then gradually forget about it.
With the risk of pandemic, we've been fortunate that some of the feared outbreaks in the past never materialised. SARS got contained in the far East. Avian flu didn't develop in to human-to-human transmission. Swine flu did hit us, but had a low fatality rate so was just a "bad flu year". Ebola got contained in Africa. So I guess it was easy to think when we started hearing about Covid-19 in China, that this was going to be SARS again, something that got contained after a few months and never impacted us. Nobody has been through a serious pandemic before as we haven't had anything on this scale since Spanish flu a century ago, so nobody had any fears lurking in the memory. I bet if Covid had come about say in the 1970s or 1980s, the older generation then would have been the ones who were taking no chances, after having memories of what happened with Spanish flu when they were young, and the "boomers" who would have been in their 30s or 40s would have been more dismissive then, just like they are now they are in their 70s or 80s.
Another thing that has made it harder to get the message across is the way politics has been run in recent years, with a "counter-intellectual" culture being encouraged where "experts" are dismissed as being part of some kind of establishment elite with an agenda to undermine Britain, and "the man on the street" knowing best. So when scientists and health professionals started saying this is a big deal, a lot of peoples default reaction was to say "what nonsense, you're not going to stop me going out for a pint".
I think most people - including the older generation - thought until a week or so ago, yes this is a new virus and I will probably get it but it will be a few days in bed then recover, just like getting a normal virus. If we had locked down earlier, most people would have thought this is a load of fuss over nothing and not really complied. The rising death count is probably making people realise that this is a real problem and so compliance will improve as people understand it is necessary.