Oh, sure! Pin me down!
My theory all along has been that they haven't released a sketch because MK didn't get a good enough look - because of darkness, mask, angle, whatever.
After the Boylan interview, I took her carefully worded comments to mean that MK did see enough. BUT, we still don't have a sketch, so there must still be a reason.
Maybe initially to protect MK but once they figured out who the kidnapper was they were able to take him into custody right away on unrelated charges and are waiting for Elizabeth or her body to appear to press charges.