In general, because there is something about those servers that it would like to safeguard.
For example, Stromm is supposed to be a newbie server. An influx of old, top-end characters with all their stuff, would cause a massive shift in the playerbase.
The reasons for not allowing transfers off are a little more unclear.
Other servers have different rulesets. It's blatantly not fair if someone can build their character up into an almight beast, then transfer to a Zek and compete against all those people who have clawed their way up PKing and being PKed all the way.
It's not that long since we had no transfers at all. 'Twas against the vision, y'know.