It was on-topic, had attracted answers, and mine was accepted (from memory, it's not accepted now).
While it might have made sense to migrate the question soon after it was posted, or if it was off-topic, it makes no sense to migrate an on-topic question after an answer is accepted.
Of course, I would not be mentioning this if not for the fact that the rep from my answer has also disappeared. That was 1/4 of all the rep I had earned here! This is not what I signed up for. The Help page What is reputation? How do I earn (and lose) it? makes no mention that rep can be lost in this way.
And then the history has been erased. On my user page, there is no mention that I ever answered this question, gained that rep, had the answer accepted, and then lost the rep.
I am, to paraphrase, mightily annoyed. Rep is meant to be the measure of our value to the community. To me, the History community has just said that does not value my contribution.