While you may know my thoughts about Purdue being afraid to promote itself, I'm guessing it was a cost/benefit. The average state fair day sees 50,000 people? And that's every single person that comes in and out of the gate - even if it's a 20 minute trip to go see one thing - or someone coming just to see a concert. Bringing all of those people and resources down there for that one day is probably not worth it, I can understand that. The question is - what are they doing instead?
I also would say state fairs have changed over the years - it's much more about entertainment and such more so than the "educational"/career type of things now.
Having one day be Purdue day is probably not worth the resources (not just money).