Because they are cruel,heartless bastards, whose sole purpose is to make the wealthy more wealthy, achieved primarily by funneling public money into the hands of their like minded mates.
Our voting system sucks, the media on the whole leans towards the right wing, and in general the population isn't very politically informed / literate and is easily swayed by fearmongering and misinformation. Plus the good old fashioned selfishness of the people who are well off or comfortable.
In the last General Election (2019) it was pretty much about who could deliver Brexit. Boris campaigned on getting Brexit done no matter what, while the opposition leader Jeremy Corbyn couldn't give a straight answer on his position (his base support were strong remain, whilst Corbyn himself is anti-EU so was in a bit of a pickle). So the Tories won many traditional Labour voters who wanted Brexit done.
Because they think austerity works. They've been making cuts across pretty much every department in the country ever since they came to power, way before Brexit.