The theme songs are nothing more than a marketing tool to promote new albums, nothing more. It appears the only criteria WWE uses when picking the songs is that they
1) come from new albums by big-name established artists
or
2) are new bands' first singles that will be pushed heavily by labels/radio.
In that sense, it's not really a conscious search for a 'great' song that 'fits' the show; it's a way to get extra $ for shilling for popular rock bands.
BTW, Saliva now is tied with Drowning Pool for performing the most PPV theme songs with 4. (No Mercy 01, WMX8, SS02, and UF04.)