Internal Shit vs. External Good
If an idea is shit but it was developed by a company internally, they'll do what they can to make it. If an idea is good but not great (but better than the internal idea) and it came from an outside source, the company won't have any interest or will look for reasons to not do it.
Your idea can't just be okay. It has to to be so good that a company is willing to overlook the fact that they didn't come up with it themselves. That's what you're up against.