Perhaps a higher percentage of 'noobs' use them, perhaps the fact everyone's lists are designed to kill Marines as efficiently as possible, perhaps multiple reasons.
Going back to this point, the stats overall don't tell the full picture. Marines suffer on average because so many people take them and a lot of them are casual or 'noob' gamers.
Also, as has been noted, everyone tools up for marines. Including.....other marines! So effectively they're a victim of their own success. Even with Tau and Eldar becoming a bigger perceived threat, marine players are loth to pack in the type of weapons to take on Eldar etc. as they usually lack the AP to take down other marines. Even missile launchers are scorned in favour of lascannons still, despite being cheaper.
So, if there were less marines about they would do better as the metagame would shift from being a marine fest. Last 2 GT seasons I've played 3 games out of 24 which haven't been a marine variant (imperial or chaos). I don't really expect that to change this year.