Because we find that many science topics can easily get heated and/or anti-Christian. Making it request only helps us control who goes in, so we can better maintain civillity in the science forum.
As a side note, most of our Terms of Service, and restrictions of forum access (science, teens etc) and restricitons of certain doctrines, such as RC, serpent seed etc have evolved over time, and there once was a time where these things were not restricted. It created much division amongst the community. So these rules and restrctions had to be put in place.