Because, Christians already like given their faith to belive in Jesus Christ, but if he's not real, then that would be tragic They want to have someone to belive in that could make their lives better and take care of them in the "afterlife" if there is one. Non believes don't really care since they already kinda know he's not real If today's the were to find out, there would be more Christians then ever.
Baiscaly more Christians+no Jesus=less Christian believes
Non believes+ a Jesus= more Christians