system
A chat on subreddit /r/NoStupidQuestions
human
Do you mean why do people seem to lean left while young and become more conservative as they age? Its actually a pretty common misconception, the most recent data shows political ideology is pretty stable over time.

Or do you mean why are millennials and gen Z more progressive then gen x and boomers?  Id say because social justice issues being more mainstream have opened the door to other more progressive support. They might think, hm I might have a gay friend or a trans friend or a black friend and I see that one political party at leasts accepts their existence or pretends to care they are getting shot in the street, while the other party still has members trying to ban gay marriage and thinks more money to cops is a good idea. Growing up with the internet the way it is was probably a big part also. 

Another big part is that the cracks are really starting to show here. Republicans LOVE to sacrifice long term prosperity for short term political points. That trickle down shit isn't exactly working like how old Ronnie said it would but at the time and shortly after there were some middle class gains. But because the working and poor class always absorb the real cost of austerity and tax cuts, it's really starting to bite us in the ass. I knew in like 2013-14 as a 25 year old that buying a house was probably never going to happen. I saw Republicans want to take away Healthcare from us. I saw them and all of us being warned about the dire situation of the planet and not only do nothing, but actively fight against any small measure to even pretend like we are doing something. I saw billionaires wealth explode while ours just stagnated AT BEST and lost value at worst with this inflation. And its pretty likely that gen z has noticed all this as well and at an earlier age and they go, hey, this fucking blows, oh Republicans want to stand in the way of every possible path to bringing us any real hope? Ill pass.
gpt
Also ignoring country politics, but one that includes the US as well.

Right wing politics can only go further to the right because their pitch involves allot of fear, fear of change, fear of others just fear of something.

The new “center” just includes more people naturally because the right just keeps in moving the goalpost.

Left wing politics hasn’t actually changed that much regardless of your country.