Rocksteady are certainly capable, and I don't why they wouldn't be willing to, unless they had some severe constraints. Would've been a far better port, like the first two of their Arkham games.
IIrc Rocksteady has close to 100 employees which is a small number for a huge game like AK. My guess is that they didn't have the resources to develop a PC port alongside the console versions or it was a low priority for them so WB decided to outsource it to some cheap D-tier developer