Your home world is dying.
A genetic mutation has caused the population to become allergic to hessian clothing. Your people are dying cold and naked in the streets or scratching themselves to death.
Your scientists’ solution? Find a hardy indigenous people who like to ware sack cloth and breed with them.
So where could they find such a people? Where else but Medieval Earth.
What they did not consider, however, was that the witch hunters of the day might not be too impressed by your arrival.
All of this is probably not true and definitely not important.
Simply teleport in, run about a bit, convert some followers, build a henge, and teleport out.
Nothing could be simpler, except for those pesky inquisition types.