I guess what contributes to Marshal being popular is his personality and design, not so much his house although sometimes people do want him to use him for the sloppy set, although that's kinda redundant now because villagers don't directly sell sloppy furniture to you, it has to be achieved in retail.
The reason why I think his personality is what contributes to him being popular is because of the fact he's a smug, I think if Marshal was a jock he wouldn't be as popular here as most people can't stand the jock personality. If Marshal was jock his popularity would go down a lot trust me, the only jock who's really made it to tier one are villagers like kid-cat and genji because most people can't stand the rest. Anyhow, smugs tend to be affectionate to the player and that's what most people like about them, because before smugs became a personality villagers would never really directly flirt with the player, and would just see them as a friend. I guess that appeals to a lot of people because of the fact some people ship themselves with their villagers, and it just makes it more canon I guess, because they actually hint at being interested in the player.
His design also contributes to this because I know some people on this forum prefer more simple villagers like myself, I prefer villagers to look more natural and although Marshal isn't the most natural squirrel he does have quite a simple yet cute design which a lot of people prefer rather then over the top villagers, such as Avery or Sterling, because Marshal can fit into almost any theme because of his neutral colour scheme. Also I think because of the type of animal he is that appeals a lot more to people because of the height differences between animals such as bears and cubs, and a lot more people find smaller animals cuter, especially at points where squirrels look almost hidden in places such as flowers. Personally, I think the taller the animal the more scary they look and that's why Marshal probably is as popular as he is because of the friendly look he has to him.
I can't believe I wrote this much about a virtual villager, but y'know this thread is just one of those threads that can invoke discussion.