I’m not sure why either. Both universities do have similarities – large upper tier state school (UIUC = 57k students, Purdue 47k), strong engineering, relatively small town, though UC is twice the size of Lafayette/West Lafayette (well, town size might do it.) And of course there’s a small sports rivalry. (Football teams play for the cannon every year.)
UIUC is closer to Purdue than Purdue’s archrivals Indiana, too, so there’s that.
It’d seem like Northwestern would be the natural choice, being in Illinois too, but it’s a private school and it’s actually farther away than Purdue. Which is sort of amusing to me.
This sort of thing can be arbitrary … I found the Champaign campus quite beautiful when I visited it & it’s been nice to see how West Lafayette’s campus has gotten nicer over the years.