Three simple, non-conspiratorial, reasons to bring up Andy Murray when discussing* Federer, Nadal and Djokovic:
1 - He can beat them at important matches at times, including slams.
2 - He is the highest ranking UK based player so they're obviously going to talk about him. The Americans are living vicariously through him in a way given their lack of top 10 players.
3 - He is ranked immediately below the multi-champion trio and has separated himself from the rest of the top ten.
* People usually discuss these guys in the context of a current tournament or slam, the press even more so. in this context you have to consider him even though he doesn't merit a dedicated chapter in the history of tennis yet.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM