If there is a deeper reason for not doing it, I'd love to hear it.
Well, what I'm about to tell you, I experienced personally in several scenarios with both Nikon and Canon.
At one point, I was told by somebody at Canon that an upcoming (then) camera was going to offer DNG as an optional file format. But something happened between Adobe and Canon that made Canon angry (which I can't disclose) so Canon decided to screw DNG (that was pretty much a quote). Since that time, some relations have improved but Canon is of the mind that they will simply not kowtow to Adobe and DNG. If it becomes and ISO standard, they would reconsider and likely adopt it as part of a new TIFF-EP. Note the "likely", no guarantees...
In terms of Nikon, there have been several cases where Adobe has really pissed off Nikon (and certain people at Nikon) and not on purpose...one of the situations was the white balance encryption situation with the D2X where Thomas rejected reverse engineering the white balance because it was encrypted and by contract, he was not allowed to decrypt stuff (which has DMCA implications), just decode it with does not have any DMCA implications. That left Nikon in the situation where they had to blink and offer a mini-SDK for decoding the encrypted white balance info. Nikon lost some serious face in that one.
The original situation happened because Adobe (read Thomas) actually had the audacity of decoding not only Nikon's NEF files in the first place but also Canon's raw files. What this meant is that a user could use Camera Raw to open both Nikon and Canon raw files and did not need to worry about using the camera software. This leveling of the paying field actually had an impact on an initiative that Nikon almost pulled off. I won't mention the players, but the initiative would have been a major coupe for Nikon and the fact that there was one software that could open both Nikon AND
Canon cameras ruined it.
But, as far as I know, the bottom line in both Nikon and Canon rejecting DNG comes down to the simple fact that they just don't WANT
to adopt any standards. Standards in Japan are an entirely different concept than here in the US where standards are thought of more as a good thing. Standards in Japan are very, very difficult to produce with any hope the standards will be followed. So, Nikon and Canon will continue to drag their feet for as long as possible (which I think is one reason the new TIFF-EP with DNG is taking so long–although I don't have facts to back that up).
There are some Nikon and Canon competitors who have adopted DNG, and that actually further deepens the resistance to DNG. Within the two main companies, there are departments that act as fiefdoms; at the top of the food chain are the departments that are producing the cameras and the firmware...from what I gather the resistance to DNG does NOT
come from from them and they may actually perfer that new cameras were supported of the bat. Then, there are departments that control the processing software...and it is from these folks that the greatest resistance comes. Even though a good 75-80% of all raw processing is done in ACR/LR, the camera software still likes to claim that only in their software can you get the best results. It would not surprise me at all that refusing to adopt DNG is largely because the camera software guys don't want to make it any easier on Adobe (Thomas) incorporating new cameras because that give the camera software a period of exclusive processing till Adobe adds the cameras. It forces people who buy unsupported cameras to at least try the camera software for a period.
Ah...the stories I could tell...maybe some day I'll name names :~)
All told, DNG has been a really good thing for the industry because if nothing else, it helped teach camera makers how to create raw file formats. Camera companies really didn't have much experience or knowledge in developing file formats before releasing their first cameras. Prior to the current CR2, Canon used .TIF and .CRW. The .TIF actually caused a whole lot of problems for Canon and it's users. When Canon first shipped the 1Ds, they wrote raw tiffs...the only problem was any software that could read tiffs could open the raw file, extract the preview file which was tiny and the raw file could be written over if the user saved the tiny file as a tiff. I remember the look in one Canon guys eyes when this was demoed to him. It was a look of abject TERROR! Hence the development of CR2 for the next camera :~)