Because there's no need to migrate it. It's a question that's perfectly good here and had answers that were interesting and relevant to this community.
If you want a canonical version on MSO for the site-wide FAQ (though I'm not sure there's a point - it's not exactly frequently asked), you can ask it again.
Grace Note is adding this part here, as another Community Manager. ♪
Per-site-metas exist as a local support channel for people to voice concerns, interests, and thoughts about their own site. This existence is two-fold - to allow users of a community to stay in their comfort zone when reporting these things, and to provide an avenue for "smaller" issues to get resolved if escalation is unnecessary. That something is network-wide doesn't preclude the fact that it is still relevant to the community in which the request organically arose.
As a result of that "cross-topicality", migration from a per-site-meta to Meta Stack Overflow is something we'd call "Escalation". That is, we perform this action to improve the eyesight on the feature in order to help get it resolved and handled. For feature requests, this means appraisal of feasibility on a wider scale, as well as to fish for alternatives/modifications of the initial suggestion from a larger audience. Support and bug questions would instead be helpful as more eyes means more analysis of what might be the source of the problem, as well as to spread awareness of the issue if it cannot be solved locally. All of this is dependent that the extra views are helpful to get that thread resolved. For the benefit of the question, which, coincidentally, is the philosophy behind migration on parent sites normally.
The thread in question, however, became resolved already. Not only did the local moderators show up and give their own perspective, but a Community Team member (Anna, here, as it were) also happened to show up and give our own official stance. The need to migrate the thread is thus negated - it's already gotten the closure and exposure it needed. There are many times that a discussion being moved to Meta Stack Overflow is beneficial to the nurturing of that discussion, but this simply isn't one of those cases.
So at the end of the day, it falls down to the fact that Metas are allowed to jealously guard their questions too. In being asked on a local site, a question develops a local flavor and appraisal. If that isn't doing the question justice, then moving helps, but if that is sufficient, moving the question is just taking away from the community. You force users to visit a separate realm just to find answers to a question that not only arose from their own ranks, but also got settled within their own ranks.
I'll repeat Anna's sentiment. If you want to spread awareness of something, you can just open up a new copy of the issue on your own site. There is no need for migration.