Because the globe theory is the best there is. Namely:
1) it is not proven false
2) it explains pretty well what it is supposed to
3) it is relatively simple.
None of that screams unequivocal certainty to me.
My opinion is that the goal of any small powerful group aiming to take advantage of a much larger, but disadvantaged group is to make them feel small, or insignificant. You see it a lot in Soviet and Chinese architecture. To convince everyone that we are a happy little accident on a speck of dust in a Universe too vast to comprehend is really the ultimate realization of that. In the very least, it seeks to remove any empowerment one may gain in the knowing that he is a creature of divine origin, that is very literally a part of The Creator himself... whether or not you share this belief, you must admit it still must have an effect on the Psyche. If there is a power to be had with the true knowledge of our lower selves and our higher selves, then it would be advantageous to attempt to horde that knowledge, don't you agree?