So MTV went from being the biggest and most widely known music channel to a cheap reality show mess. When exactly did it become a non factor and why?

All the reality TV shows.

Dumb ass shows.

16 & preteen
Jersey shore

& not airing music.

Real World is the first major reality show I remember from them, so I guess it started there?

Honestly when the music industry fell so did MTV. So basically people who were illegaly downloading music

The realities & tv shows

16 & preteen?!?!?

This basically.
oh and Teen Mom.

When YouTube became the go-to for music videos.

Spencer Pratt

When they started competing with VH1 during the Flavor of Love/New York/I Love X reality tv days, I think.

Youtube was the reason behind the fall of music on MTV and reality shows were the consequence.

I believe what happened first is that less people were tuning in to these music video shows and just opting to watch them on youtube.

Then, when MTV saw the ratings decline with MV shows and ratings increase with reality TV shows, that's when they switched.

Reality shows and tastelessly sticking to them. I don't completely blame MTV, though, the same people who complain about it not playing videos anymore are the same ones who sit through re-runs/season premieres of the same insane shows and make trending topics about them.

MTV was long dead before Vevo Crap Realty shows ruined it.