Why did Democrats let their party be taken over by leftist extremists?

Liberals don't have any deep rooted pro American beliefs like Conservatives do . They have their feet firmly planted in mid air and can go in any direction as long as it is not in agreement with Conservatives .  Don't they know that history teaches that leftists lead countries into bondage ?

You do realize that what's considered extreme left in the US is considered center/moderate in practically every other developed nations, and those nations have far better qualities of life than the US, which is at an embarrassing 15th in the world. 

They didn't. Bernie Sanders is a moderate leftist

I don't think there are any anarcho-communists in the Democrat party

Simple answer::: propaganda 

It worked do well for Hitler when  Germany was still getting over WWI, the crash of ‘29 

Fast forward::::propaganda it’s in the lefts interest to mirror that time 21st century style 

The virus and a bunch of ppl that don’t realize how good they have it

Not true, if that was true Bernie Sanders would be the Democrat Presidential nominee not Joe Biden.

If that was true then Bernie would have won the nomination not Biden. Bernie is alot further left on the political spectrum. 

No. That’s why we nominated Biden. You let your party be taken over by alt-right extremists when you nominated Trump.

Democrats are centrist to left leaning. If you are right wing, vote for the other party.

I disagree with your starting premise. Establishment Democrats are more center-left. People more to the left (AOC, Sanders, etc) are often at odds with these people (Pelosi, Schumer, etc) 

Why did Republicans allow a maniac like Trump to be nominated?

Many actions occur without logically reasoning. I see it as reactionary to the Trump-McConnell and their massive catering to the rich and corrupt administration that caused a left wing upstart, but Biden is less radical.