Population shift isn't the only reason.
Coaching is a big part of the reason.
Look at all of the recruits that are leaving the Midwest and going to southern schools.
it used to be that student athletes that were elite but not quite MI, OSU, ND would go to the other B10 schools or somewhere Midwest. Now they are heading south.
Kids that used to go to the second tier of the B10 are now headed to the second tier of the SEC. Kentucky is cleaning up the Ohio leftovers that used to be taken by Purdue, Indiana, Minny, etc.
The SEC pays their coaches and assistant coaches quite a bit more than the B10 does. They want excellence and will pay for it.
The B10 doesn't pay for their coaches and you see the results. If you are a good coach at Purdue, Ill, Iowa, Wisci, etc you will be paid more to go south or west.
A power like MI brings in a career 500 coach..Puhleese.
Weather probably plays a small part. However, if you listen to recruiting analysts, they say it is a very small part.
Most of these guys going to the big time programs want to play in the NFL and know they will be playing in cold weather. They can't be afraid of it.
If it meant that much, Hawaii would be a power, as would NC and UCLA and San Diego State, etc.