If the producers of the show want to use it as a platform to produce Christianity, or Islam or Buddhism or any other religion they see fit, they are all within their rights to do so. Every show, whether overtly or otherwise, promotes some values over others. There is no reason why Christianity would be off limits.
No. Although you see the Simpsons going to a Catholic church in the show, this is only because the national religion of the US is Catholic. If we were in another country, I would expect their culture would dictate the religion used on the show. The Simpsons don't promote any religion in particular. If anything, they poke fun at organized religion.