Musical inbreeding, encouraged by the industry which is convinced that there's a formula based on recent trends. But there's plenty of good off-mainstream stuff to hear if you do your homework and there's bound to be someone refreshing and good to come along sometime (probably not soon enough) who will be popular.
Why does all new pop music sound the same?
Basically because it IS the same - I think anyone with ears already knows that, but there is more to it.
https://www.youtube.com/watch?v=oVME_l4IwII
https://www.youtube.com/watch?v=oVME_l4IwII