I'm gonna guess that they couldn't figure out how to get people thru the van Allen radiation belts without frying them, that's my guess. And that's my guess as to why the space shuttle never left the safety of the earth's orbit
Because its much, much easier just to stage the whole thing and win the
moon race without leaving the starting line. But not only the
moon missions but subsequent Gemini 10 and 11 space walks as well, because those allegedly took place right in the Van Allen belt, indecently those space walk photos were faked by NASA. But the majority of the space flight missions, well over a hundred, stayed out of the southern belt region all together.