Because young people are fickle and Trump realized very early that they wouldn't be a worthwhile vote to cater to for a midterm. He won them when they were guaranteed to turn out (during a presidential race of epic proportions). He then catered his entire Presidency to other demographics, and poor analysts screamed at him for doing this. But the why was always obvious and it's manifesting now. Young people don't vote in midterm elections. Even if their preferences shift left, nobody needs to care because they wouldn't show up anyway. Poor analysts claimed they'd vote left, but failed to realize that the Democrat party was competing with terminal cancer for likability per every poll over 2 years. So why would low-propoensity voters decide to vote for an extremely disliked political party? They wouldn't. Instead, they are absence-maxxing.