Musicnotes has one of the largest catalogs of Christian sheet music available online - including a lot of music that is no longer available in print. This large catalog, along with the fact that Christian music is much more "playable" as well as has a stronger emotional tie to the musician is part of the reason (in our eyes, at least) that so much Christian and faith-based music makes it into the top ten. The music is very well done - and people genuinely care about the music and the message... which is not something that could necessarily be said about most of the junk you find on top 40 radio.

Apparently there is a market for listeners who like to hear music with a "feel good" and "positive" message. I'm surprised that Taco Bell's (pachabel's) Canon in D made the cut. Probably downloaded for weddings for the summer.

Speaking as a musician here, my guess would be that Christian or faith-related music has more venues of being played and usually when a song is needed, this type of service where you only have to buy the one song (and not the whole book) is super handy. This list looks like songs that would be used at weddings as well.