NVidia can get away with what it does - producing a binary module with a GPL wrapper - only because its code doesn't need to touch too much of the kernel and doesn't need to modify core routines.
That's not true of DTrace, which must be deeply integrated throughout the kernel. Doing it as a binary module would require vast numbers of currently-unexported symbols to be exported (creating bloat) and would require many GPL_ONLY symbols to be exported w/o that flag, something that often can't be done without the agreement of the authors of that part of the code.
So: As things stand, licensing issues prevent integration of DTrace into the Linux kernel.
As for the compat issue being caused by the GPL, not the CDDL: Yes, that's true to an extent, in that the GPL is a painful license to use other code with. However, it's entirely reasonable and possible to dual-license code (including CDDL+GPL, see Glassfish etc) and Sun could've chosen a GPL+CDDL license that would've permitted all current uses plus integration with GPL code. They chose not to because, for commercial strategic reasons, they wanted ZFS and DTrace to be differentiators between Solaris and Linux. Fair enough, really, but it was/is still very much Sun's choice to create the license incompatibility.
Even were that not the case, it's not clear that DTrace could work as well and in the same way in Linux, given the lack of stable internal kernel API in Linux. It'd probably have to be treated like DebugFS, ie "not part of the API, things may change name or break without notice".
License and techical issues aside, though, there's certainly a fair bit of NIH mentality in the Linux kernel circles. Some of it seems to make a lot of sense, in that you'll get a much cleaner system if you're not integrating code full of compatibility adapter interfaces and unnecessary layers of abstraction. Also in that sometimes "someone else is using it this way over here" isn't a good enough reason to use something - for example, if it's crap designed by a collective of baboons with keyboards. Some of the issues, though, do seem a lot like pure and simple NIH.
I don't think that's the case with DTrace, though.