"We come into this world alone and we leave the same way. The time we spent in between - time spent alive, sharing, learning together... is all that makes life worth living." - Jean Grey
I love Mann's art, it's absolutely beautiful - but I don't want him anywhere near a monthly book unless it's on covers.
Batman/Catwoman was delayed nearly a year, in part to "guarantee" he could do the whole thing. We all know how that turned out...
His artwork leaves me hot and bothered, but MY LORDT he is sexy in person as well!
A lot of these artists are at a point in their careers where their page rate / cover rate is good enough that they don't have to wreck themselves to keep to a monthly schedule. The level of detail artists like this put in takes a lot of time and labor, and producing that on a monthly basis isn't sustainable in many different ways.
There's also the fact that the single greatest reason to do interiors is out of personal passion. Comics interiors do not pay very well, even for the big name artists. Especially not work for hire. Cover artist gigs are much better compensation. Art gigs outside the comics industry are even better. Comics is a passion-driven industry. A seasoned veteran of an artist isn't going to have that same wild abandon.