1) Bigger and better city (at the time). Good enough location that it warranted interstate access years later.
2) Interstate access itself is an advantage, now that it is built. Jackson 1 hour, Birmingham 2 hours, direct routes.
3) Proximity to the Coast and the 3rd biggest city in AL (Mobile).
4) Fun. Well documented that they wanted the cadets to NOT have fun thus they located the school in Starkville.
5) Farther from Oxford and Tuscaloosa.
6) Meridian is under-served enough that we put a secondary campus there.
7) USM would have never existed most likely.
8) Meridian's downtown would be very nice and apt for a college town if it had been kept up.
Think of all the money that has gone into the GTR complex, that all could have been spent making Meridian bigger and better all these years, and/or Tupelo area. MCC would have been more perfectly located in Starkville, then we would not need the EMCC-Mayhew campus. The entire country is going urban. Mississippi's decision to create small towns all over was not a good one, as were many decisions made in the 1800s.
Shall I continue? It obviously does not matter now, we must do the best with what we have.
Edited: I see now that you are asking why Meridian was best, not necessarily a comparison to Starkville. That still does not change my opinion. Some people say Jackson would have been best, but while I do not like the idea of a bunch of little towns, Meridian was a well-established city and I think it is important to separate your state's biggest city(s) from its college towns. College towns should be unique. And please do not say Hattiesburg. Just.....no. Coast? See my reasons for not locating in Jackson.