Popularity rankings, which determines if your work could potentially get axed or not.

Let's say for example, in a certain magazine, ten authors are getting their works published. Then within ten readers of that magazine, six are of a certain demographic. It would make sense to tailor your work to suit the tastes of those six to maximize the potential fans that you would gain, right? But that's not factoring in the rival authors who are also aiming their works towards the same majority demographic. But if for some reason you're the only one who tailored their work to suit the other four, you essentially have a monopoly of 40% of that magazine's readers while the rest are fighting against each other to get the biggest piece of the remaining 60%. Just because the magazine is aimed towards a certain demographic, it does not necessarily mean every single reader of that magazine is of that demographic.

If mangakas could get away with just drawing whatever they want all the time, they'd be better off staying as an amateur and make doujins, or self-publish. Both of which aren't exactly as profitable as a professional in a big publishing company.