I feel there is a common audience trend in favoring the side characters more than the MC. And I believe "literary science" backs that up, in regards to character focus. That even though the MC's character and development is very good, it is harmed a little by the great focus on them, due to the audience becoming too familiar with them, and therefore bored.
With side characters, there is less familiarity/more mysticism, and less predictability. More wonder, perhaps?
Or maybe, MCs tend to simply be written worse? More boring and standard. That seems to be the case with Harry Potter (never read the books or seen the films). I have been told by many that Harry is actually the least interesting character in the whole work. The most boring character, and perhaps more boring MC in all of mainstream entertainment. I've also heard a lot of people favor all the side characters in The Walking Dead over Rick Grimes.
Conversely, speaking to a family member, they said they absolutely love Michael Scofield, the MC in Prison Break. Of course, they are not a critic, and furthermore, my whole question is spawned by the fallacious argument that "because it applies to some/all people I know/have observed, it is true for the rest".
But it might be that most audience members do like the MC the most, as one would assume considering they are the main character, who usually gets the most focus, development and generally care that is needed to create a compelling character.