I don't care about sales numbers. I don't take them as an indicator of value. For example, Alanis sold 20 Million copies and she can't sing or write songs. Another case in point is that Gary Puckett and the Union Gap supposedly sold more records in 1968 than the Beatles did.
More importantly, the numbers don't tell me what I was asking - which is why people like it, what makes it resonate. Whenever I ask people, I rarely get a more thought out answer than "Oh it's so good, how can you not like it".
I also have to add that having great musicians on a track does is not a sufficient condition for a great track.