tl;dr It was the simplest way to take the questions off Main.
I've wondered about it, but perhaps one reason is that providing resource suggestions is about how to use the site: at least where to do your own research before asking a question. – Andrew Leach♦
I didn't find any reasons on ELU Meta for resources to be on-topic there. I also looked briefly at Stack Overflow for precedents (their analogue is the recommendation question) but didn't see any reasons there either. Since it appears that even Andrew Leach doesn't know, I'll assume there is no canonical answer, and present some speculations and suggestions of my own.
When considering recommendation questions on SO, Shog9 identifies the problem of providing canonical answers to questions that don't ask about how to solve a specific problem. Elsewhere, Robert Harvey notes that "we don't accept questions of this type on any Stack Exchange site".
This tells us why resource requests should not be hosted on Main. However, the resources elicited are useful and interesting, so it is natural to want to keep them. I speculate that they have been migrated to Meta because it was the simplest way to take them off Main. I further speculate that because there aren't all that many canonical resources to list, having a small number of off-topic posts on Main or Meta (both, actually) has simply been tolerated. They are useful, and they don't really get in the way most of the time.
Nevertheless, I agree with your view that resource requests are off-topic on Meta. The question is what to do with them, since I don't think the community is seriously considering just deleting them.
Of the four spaces available to ELU: Main, Meta, Blog and Chat, the Blog is the only repository of filtered ELU wisdom. I propose that resource suggestions be discussed in Chat and on Meta, but eventually collated on the Blog and referenced from the help pages. The existing Main and Meta questions can either be retained for their historical value or replaced with stubs. Either way, those questions should be edited to reference the blog(s) either directly or via the relevant help page.
In the unlikely event that the list of recommendations (the 'answers' part) grows beyond casual pruning and additions, we can consider launching our own version of SO's Software Recommendations.