I know, but has it been said officially that he choose them? Satriani? Vai? would Mark choose them?
My guess is that with ongoing sessions this thing got its own life and people started to ask if they can participate. All MK had to do was saying "yes of course"
LE
Those are great players anyway, and you need to live under a rock to not appreciate Satriani, Vai, Slash, Iommi and all the rest of 'em, even if you're not a fan of their music. I never purposefully listen to these guys, however, I know their hits, their signature songs and their style. The best part about it, is they surely have some deep mutual respect towards MK, as his precise finger-style playing and intricate lines are as difficult, or often even more difficult than stuff these guitar heroes are doing with their picks. And he's doing it all in his own brilliant songs, without stop for 50 years.