Part of the problem that Purdue faces with respect to keeping graduates in-state (and with cultivating a larger fan base) stems from the large number of international students on campus. For the 2017-2018 academic year, Purdue enrolled 9133 international students (21.9% of total enrollment); for the undergraduate student body, 16% were international students (with China accounting for the largest number by a considerable margin). Purdue is third in the nation among public universities in international student enrollment, and second in the Big 10 (to Illinois). On one hand, international students are less likely than their American counterparts to attend athletic events during their time here, and thus less likely to form memories and loyalties that will make them athletics boosters (and contributors) once they've graduated. And, of course, once they return to their home countries, they are not likely to be in Ross-Ade on Saturday afternoons.
Now, another layer of the dynamic at work here is that international students pay a premium to attend. For 2017-2018, in-state tuition was $9,992; for international students that figure is $30,954 (out-of-state students pay $28,794). This is part of the reason that Purdue has been able to flat-line tuition for the last few years. Ten years ago (2007-2008), international student enrollment was just under 5,000 (compare that figure to 9133 for 2017-2018).
So, Purdue finds itself in something of a dilemma, at least with respect to keeping tuition costs relatively low, versus nurturing a larger fan base.