Because Glee is about High School kids and their problems and kids relate better to kids?
Which 14 year old is primarily interested int he affairs of a mid 30s teacher?
As to Kurt:
He started to get grating after Ryan Murphy started to make him his gay centerpiece, his loudspeaker so to say. In general it's not bad to have homosexuality as a theme because many young teenagers who are seem to struggle with it but as the seasons went on sometimes i got the feeling it was all about homosexuality.. they even "turned" the hottest cheerleader into a lesbian!
It just got a bit much for me.