Considering that there are over 1300 open questions with the data science tag on Stack Overflow, I'm not sure that it's fair to say that no data science questions are on topic there. Remember, sites can have overlapping topics. That said, there's a request on Meta Stack Overflow to get rid of the tag entirely:
Burninate the [data-science] tag?
With so many questions using it, though, that takes a lot of teamwork to clear it from existing questions or retag questions that need a better tag to prep for it being removed.
I'm not personally a fan of this but my understanding is that, in some cases, Stack Overflow does do this, such as the
history tag, which currently reads:
That said, there are very good reasons that migrations are limited the way they are. For a migration to go through several things are required, as mentioned in the FAQ for migrations:
- Don't migrate crap.
A subject being generally on topic elsewhere doesn't mean that all questions should be move there - only good questions.
- Question must be less than 60 days old (even for moderators).
- Questions may only be migrated to sites that have migration paths and each site can only have up to five paths leading away from them. Explanation is here.
- No migration paths are established to or from Beta sites.
This is because beta sites are expected to find their own community. A huge influx of questions from an established site will overwhelm a new community and they will be unlikely to manage it. As such, only moderators can migrate to beta sites.
- Questions must receive at least four votes to migrate (out of five total close votes) in order to be migrated (non-SO sites it's only three).
I'm not quite sure why you think more votes are needed to migrate than to close. Close votes (for any reason other than gold badge duplicate votes) require five close votes. Migrations are a type of close vote. Also, as I said, out of the five close vote slots, only four (on SO) need to be migration votes.
So, please, don't flag stuff that's older than 60 days for migration and avoid flagging huge numbers of things for migration to new beta sites. In the case of Cross Validated, it's possible that many of the questions were already asked there, so migrating large numbers of questions makes more work for them to figure that out. Bulk migrations are extremely rare as the CMs have to do them.
So, if it's one or two new questions and there's a migration target, judge the quality of the question. If it's good, vote to close and migrate to that site. If it's bad, just close the question.
If there's no migration target, and a good question, you can flag for migration. It helps if you know the target site and are able to attest to whether the question would be on topic there or not.
Old questions (older than 60 days) that are off topic on Stack Overflow should just be closed. Trying to migrate them is a waste of everyone's time.