Because in order for any industry to thrive in a capitalist society, there needs to be competition. Disney was already the top studio in Hollywood before all this, and now they’ve completely absorbed two other studios? That could have very negative ramifications for the film industry in the near future, especially if it leads to more studios shutting down.
Not only that, but this will inevitably mean that fewer movies will end up getting made, including fewer movies from Fox Searchlight, which is one of the most prestigious small time studios in Hollywood. Plus, since Disney is generally pretty keen on big franchises, this means that there will likely be less variety in the kinds of movies that get made moving forward.