Maybe a lack of resources due to a lack of commitment. Maybe all the effort to certify a diesel isn't worth it for just one vehicle. It might also be something about the design of this particular engine that makes it very hard to bring some emissions parameter below a certain level. It is rather unique in having a low compression ratio, although that is theorized to result in lower NOx so you'd think it would help. My best guess is that they thought it would be easy, just add urea injection and viola it would pass, and when it turned out to be harder, they weren't committed to investing the additional effort.
You could be right. Why bother. It's not like it's going to massively add extra sales