The Walt Disney Company owns a huge number of brands. One of their brands, and one of the most famous and valuable of them, happens to share a name with the company that owns it, but that doesn't matter much. What matters more is that it is associated in the minds of consumers with a certain kind of "safe" entertainment that won't bore young kids or scandalize their parents, and Disney (the company) wants to preserve that association because it's the source of the brand's value.
While some of the movies released under the Marvel brand may fall in that category, some clearly don't, and you can't really separate them because they take place in a shared world and reference each other extensively.
It's possible that the Marvel fan base might react negatively to a rebrand. And Marvel is itself one of the most valuable brands in the world right now, being associated with several of the highest grossing movies of all time, so there is no incentive for Disney to risk rebranding as long as the status quo continues to bring in customers.
