Because she's from the 80's, and most of her discography didn't have much longevity because to the general public, she's an entertainer. The reason why the younger generations are more likely to know songs by The Beatles for example, is because their music and songwriting was always the highlight of them. Madonna was all about the performance, and there was always much more time and effort put into her tours for example than her actual songs. She's not a good singer either, which doesn't really help. Had she been a great vocalist, her songs probably would have a slightly higher chance of being embraced by the younger generations, like Michael Jackson for example.
The kids of today probably just know her as the old lady who sucked Drake's face off at Coachella
