The big problem here is what DavidZ mentioned - questions eventually become too old to be migrated, though I believe the limit is 60 days.
I've seen this discussed before on Mathematics Meta, and the consensus was what I think should be the policy on Physics - and what I think is the unofficial policy here - which is that history questions are on-topic.
Take, for example, this question:
I would love to see this on HSM, and I think that we'd accept a migration of it to HSM. It clearly deals with history, and is, from what I can tell, 95% history and 5% physics. That 5%, by the way, is gratuitous.
The problem is, history of physics is still on-topic on Physics.
The comment left below the question, by alarge, seems to be rather typical in this post-HSM-launch era:
Might hsm.stackexchange.com be a better home for this question?
"Better" is the key word here. The comments below a history question may be laid out the same, but the word "better" is always used. History questions are okay here. Also, there have been quality answers to many of the history questions.
Besides, while we're trying to increase traffic and question rate on HSM, we don't want to artificially increase it. This was discussed a while ago, and we reached the conclusion that there's no reason to do it because these history questions have done so well on other sites. We want to "homegrow" questions and traffic - in other words, we would like to attract users and questions on our own, without this kind of artificial stimulus.
Plus, we can't handle the migration of all these unanswered questions at once - we simply don't yet have the user base* - and slowly "weaning" the other SEs off of unused history questions would be unproductive.
- I also - as a pro-tem mod on HSM - don't want to have to manually re-tag and edit hundreds of questions - taking into account history questions from all the science and math sites, as well as science questions from History. :-)