Because the young think simplistically. I don't mean that to be condescending - I mean it as a fact. "Help the poor", "save the environment". Everything is black and white, good and evil.
As you get older you see the reality - there are shades of grey in between.
The current younger generation aren't in a unique vacuum. You had the 'hippies' in the 60s and 70s who felt the same who grew older and their thinking evolved.
Or maybe the younger ones are more tech savvy so are very unlikely to be influenced by Murdoch the Barclay bros etc through newspapers and the BBC's propaganda machine. The older generation believe what they read, so if you tell them x, y and a and its in a newspqper then it must be true.
There is a similar pattern emerging in America now with Sanders, they are even laughably trying to accuse him of anti-semitism (apparently the new buzz word) when his family were killed by Nazi's!
The likes of Corbyn and Sanders are the biggest threat to the rich and powerful so the campaigns against them are like nothing we have seen before.
Unless there is major reforms in reporting, journalism and electoral reforms, I dont imagine we will see another Labour leader in a very long time, regardless of how good their policies are or how well they are liked. There is simply too much dark money influencing our elections