Are there a couple dozen, maybe a couple hundred people who buy season tickets on the home side 100% due to not sit in the sun twice per year? Sure. But its not the sole reason 75% of fans sit on the Pitt side and 25% sit on the other side. If Pitt's sideline was switched, more people would be along that sideline.
I remember I bought tickets off a scalper for the game vs 11-0 Miami game because tickets for that game were actually sky high on Stubhub so I sold them. I sat on the Miami sideline and the Pitt sideline looked very full, maybe 90% full of all Pitt fans. On the Miami sideline, it was maybe only 70% full with maybe 30% of the fans being Miami fans. The reason? The tickets on the Miami side were never purchased as season tickets or bought by brokers which makes them available to be purchased by visiting fans. Pitt fans want to sit on the Pitt side.