He has successful Flash and Robin ongoings, and despite all criticism, Dark Crisis sold really well. They have no reason not to give him more books.
And in case of Batman & Robin, it looks like Williamson is just continuing his Robin run, after Waid finished Lazarus Island story arc, but this time as Dynamic Duo story. I don't know if it was planned from the start, or it's because of Gunn, but it makes sense for Williamson to continue write Damian, especially if there are not many people, who actually want to do it.