a few reasons. economy being one of the major factors obviously (despite the fact we have the worst record and gdp drop of any developed nation).
the other is Boris and the Tories like to be seen as libertarians that do not want to involve themselves into peoples daily lives. Now of course that's all well and good in 'normal' times. During a global pandemic, you need to act like there's a global pandemic.
Ultimately, stringent restrictions early on, such as closing of borders for international air travel only (not freight goods and the like, we need food etc), and a proper testing and tracing system, we could be having 10,000 fans in packed stadiums and out for brunch.
Still, i look forward to us discussing how we have strong border controls as part of our brexit bonus, we just won't use them in times of dire need. Wonderful.