Because he just 'gets' the vast cast of Green Lantern too, I think. His work with Kyle, Soranik & Guy has been nothing short of fabulous, and the characters he's introduced -Miri, Kryb, Mongul & the Daxamites- have all been compellingly written.
Don't get me wrong, I would like to see what Tomasi could do on a Bat-book, but I really can't think who could replace him on GL Corps. It's a difficult book to write well.