Why? I think you mean "why not?"
-The first thing is that the move pays for itself. All that big TV money, bowl revenue sharing, etc will be pouring in. Conference entrance fees, increased coaching salaries, travel costs and so forth can be covered by the TV contract.
-Elite-income sports fans will finally have a local team to support. These millionaires are geniuses that require a high-quality product before they will write the big checks.
-Griz will be able to get great coaches instead of a bunch of losers who will never sniff a Power 4 job- Hauck, Smith, Doeren and those other fools will be a distant memory.
-OOC schedule will improve with Power 4 teams flocking to WaGriz with home and home deals- Oregon, Washington, USC, Ohio St... even Notre Dame, probably.
-Cost of education will go down for students as fees will be practically eliminated due to the massive influx of funds. I'm sure JMU now has the lowest student fees in Virginia, maybe the entire US of A!
-Montana will finally be competing with peer institutions in term of enrollment/endowment, like ... well I can't find anybody. Does Valdosta State, Cabrillo College and Northeast Wisconsin Technical College have D1 football?
-No matter what conference the Griz get in, the schedule will finally be filled with bigtime opponents like Western KY, Delaware, UMass, NDSU, App St - not the current assortment of I-AA also-rans.
-Most importantly, Griz fans will no longer be shamed while gathered around the watercooler in the office. That jerk from Utah St won't have anything on a Griz fan!