Another reason UM has more - look at a population density map of Mississippi.
http://www2.census.gov/geo/maps/dc10_thematic/2010_Profile/2010_Profile_Map_Mississippi.pdf
I would say a lot of north MS sidewalk fans lean toward UM rather than MSU simply because it's closer to home. Draw a one hour radius from Oxford that reaches basically to Memphis / Desoto County, Holly Springs, Tupelo, Batesville, New Albany, Pontotoc. Lot of small towns but also a lot of people living in rural areas outside those towns. You have a lot of non-grad people with decent incomes due to the manufacturing industries in this area. They can afford to go to ball games and many live within about an hour of Oxford.
Now do the same thing with Starkville. You get Columbus, West Point, Louisville, Philadelphia, Kosciusko, Houston, Amory, Aberdeen, and stretch just barely to Tupelo. So you have some small towns, but the area between those towns are bare... no one lives there. Those areas are also on average poorer than the counties to the north where UM gets a lot of it's sidewalk fans. And to the east (Columbus area) you're competing with Bama.
I went to a high school 30 minutes from Oxford, 90 minutes from Starkville. I can't tell you how many UM games I went to and how many school trips we took there. I only visited MSU one time in my whole life (when I was about 12) before I went there looking for an apartment. I would guess 90% of my friends from highschool would say the same thing. Very few went to college, and so now they are UM sidewalk fans.
Strangely... almost all of my highschool friends that went to a 4-year college chose MSU over UM.