I chalk it up to it being the 1960s.
In Simpsons season 3, in 1991, they did an episode where it was a moral tragedy that Homer wanted to stop going to church.
Ten years later they did an episode where the church sold out and Lisa converted to Buddhism.
Before the 90s everybody on TV was Christian unless proven otherwise. Around the mid nineties religion started being an aspect of a person's life rather than the defining element of culture. In the 80s you couldn't address religion on TV without flaunting Christianity. In the 60s they pretty much had no choice.