He's been switching endorsements a lot (he is playing Hughes & Kettner now)
I don't think that Epiphone attracts more endorsers than Gibson, it's just that Gibson does not build signature guitars for your everyday worldfamous artist, they only do it for Guitar Legends and Manistream artists...
I think there will be a Machine Head Epiphone Flying V soon, I saw Robb Flynn play a custom Graphic Epiphone V on the last tour!