It's a simple fact that ordinary people cannot be expected to understand the intricate details of fundamentally complicated political issues. This is why we elect people to make decisions on our behalf.
Also, tyranny of the majority should be avoided at all costs.
MPs are paid to understand complex political issues better than the average person. That's literally the reason they exist. Voters, meanwhile, do not understand these issues, are not constitutionally bound and will vote irrationally and emotionally rather than through being informed and professionally responsible.
Not with a constitution designed to stifle the power of representatives (which of course the UK doesn't have, but we're talking theoretically).
It's how radical direct democracy would theoretically work.
Does it though?
Switzerland has numerous very unusual laws in place, many of which are the result of their democratic system.