Montenegrin is probably the longest-standing issue we have in the language committee, and it has been requested numerous times through the years. In the beginning of the language committee, the nation of Montenegro itself was brand new, and Montenegrin
had already been "lumped in" with Serbian for a long time (well, at least since the Serbo-Croatian code was split up).
For a number of years, and this may be why LangCom's stance on Montenegrin has been the same, Montenegrin simply didn't meet one of the formal criteria: having an ISO 639 code. Then, in 2017, the language finally got its own code, but there was skepticism
within the committee because the code was an ISO 639-2 code and not an ISO 639-3 code. The registrar for ISO 639-2 is the US Library of Congress, while the registrar for ISO 639-3 is SIL International. For all their faults, the latter is at least perceived
to be more linguistic-minded, but many people saw the code coming to ISO 639-2 first as a sign that this was a
politically motivated decision, and not a linguistically motivated one. A single language (and not a language group/macrolanguage) having a code in ISO 639-2 and not in ISO 639-3 was a highly unusual situation.
However, it seems to me that the code was also added to ISO 639-3 later without much fanfare, though there isn't the usual paper trail on the ISO website:
https://iso639-3.sil.org/code/cnr
I have dug up previous discussions from the mailing lists that I would advice all language committee members to read through. Most of them are on the internal list, because the discussions happened before we reached consensus to have a public list. That
means they are not available to external list observers, but committee members should have access. If you don't, reach out to me off-list, and I will help guide you through the process.
1. April 2007: The thread "
Incubator and conditional approvals". It is a very long thread, the parts
most relevant for Montenegrin happen in mails 2–8, but the rest can be an interesting read too if you want more context.
2. March 2010: A single email "
Request for Wikipedia in Montenegrin" from our now-former member, User:Millosh,
who is himself Serbian and therefore knows the local situation quite well.
3. December 2010: The thread "
Montenegrin Wikipedia rejected"; it was rejected because a pending request
to add an ISO 639-3 code for Montenegrin was rejected (or not acted upon?) by the registering authority.
4. May 2011: The thread "
Montenegrin 4"; rejected because of continued lack of ISO 639 code.
8. December 2017: The thread "
Montenegrin Wikipedia (URGENT)", which came after the ISO 639-2 code was approved, but before it
was added to ISO 639-3.
And then there's the current thread.
As we can see, I don't think there was ever a clear consensus to outright reject Montenegrin. There was a consensus for many years to wait until they potentially got an ISO 639 code, but when that finally happened, it happened in what I would describe
as an "out-of-process way", which seemed perhaps "suspicious" on linguistic grounds.