Currently, Data Science site has no ability to specify what StackExchange site a question should be migrated to. This feature exists on some other sites, for example, Cross Validated. I think that this issue should be fixed ASAP.
I am not entirely sure about this but I think the list of sites you see may be a function of which sites you have some minimum rep on? For example I know the list on SO that I see is different from others. As a mod here I can migrate anywhere.
So it might be by design and at least it is something that I dont know how to change directly. I would not mind if everyone could vote to migrate to the 3 most common destinations, SO, Cross Validated and Open Data.