And as much as I don't love the idea that there's a separate, specific industry for "Christian" stuff, I understand it, too. People need to make money, and they found a market segment, and it will pay for what they offer. Or, people want to categorize the movie they made by the kind of consumer who will be glad they spent their time and/or money on it, and it keeps people who won't like it from wasting their resources.
There's no inherent reason someone couldn't make a good "Christian" movie. There's also nothing keeping a conservative or liberal film from being good, or really, any genre film. Tons of bad horror films are made for money, but there are some good ones, too. There are dozens of terrible romantic comedies, but there are some where craft and genre work together to make something that's good. Plenty of "family" films are hackneyed, but some are quite enjoyable. I really hope that there will be some good Christian movies in the future, too.
When that's not the case, though, it's for the same reasons that Micah Mattix and Alyssa Rosenberg point out: movies and TV are not just vehicles for messages.
When art is made in order to carry a message, it becomes a servant to ideology—to a system of abstracted ideas and ideals. Ideologies are not in themselves bad, but they often hit rough patches when they come out of the clouds and down to earth. (For instance, the pacifist who is suddenly less of a pacifist when the lives of his wife and children are threatened by an intruder.)
Movies and TV shows built to transfer particular abstract ideas wind up fitting the story to the ideas, instead of letting the story and characters breathe and live like real people, who are messy and inconsistent and confusing. Like you. Like me.
You believe in your ideology because you think it's the best one, the correct one. You have to. If you don't believe your ideals are the right ones, then I don't know why you believe in them.
Humans actually are pretty good at figuring out if someone is telling them a story in order to talk us into believing they're right. We hate it. But we also like seeing the results of our ideologies played out on screen in ways that are favorable to us.
But you know what else is true? Conservatives don't really like movies that too obviously try to communicate liberal ideas. And liberals don't like movies that are trying to hard to make them conservative. The market segment is all wrong there.
The ones that do work are suggested by Jonah Goldberg and Alyssa Rosenberg (a conservative and a liberal, respectively) above—stories where the story and the character are the point, where empathy is exercised. Without empathy (which is, at its core, good writing) and some careful craft, these movies sell tickets at the box office to the choir, but they don't "work" as ways of proclaiming an ideology. Because it comes off as preachy. Because it feels like being bludgeoned. Because it feels tone-deaf to why you—the not-conservative or the not-liberal—choose not to be those things.
"Christian" (adj.) films have the same exact problem: if you don't agree with them already, which is to say if you're not part of the market segment (which doesn't exactly line up with the noun, by the way), then they're just going to make you mad.