DT is a good actor, but he just didn't fit the version of Billy, as BM played him, that fans became accustomed to seeing with Victoria - who were/are a very popular couple. It's like trying to replace 1/2 of a super couple and then expecting the same chemistry to be there. But that usually doesn't happen with recasts because the chemistry is intrinsically part of each actor's interaction with the other. Replace one of them and the chemistry can, and usually does, shift. No fault of DT's, but he just didn't fit the role - either in a pairing with Victoria, or verbally going up against Victor or Stitch. The disparities were too glaring.
I wish DT well in his future endeavours.