I think that is due to the fact that Church of England is not a religion, neither is catholicism or JW's. These are all parts of Christianity. But to your question, perhaps the reason is some sort of belief. May be others are more stricter than CE.
I know that is not the part of the question but being a Muslim I know why it is compulsory for man to convert when marrying a Muslim woman. Muslims believe in all the prophets (including Moses and Jesus and that is necessary to be a Muslim) so anyone converting would not have to stop believing in any of the prophets. Whereas if a Muslim marries to Christian then Muhammad (pbuh) is out, however if she marries a Jew then both Muhammad and Jesus are out.