UIC has definitely been taking a lot of students that used to go to the non-UIUC public universities in Illinois over the past several years. UIUC has had intentionally modest enrollment growth over the past decade, Illinois State has held steady, the directional public universities have plummeted, and UIC has skyrocketed. I believe that it’s a combination of UIC’s relative strength in STEM disciplines (which more students are focusing on today), its West Loop location in Chicago going from was used to be a liability when I was growing up (when it had a legit reputation of being a dangerous area) to now being one of the hottest urban neighborhoods in the country (with the large tech firm concentration in the area, particularly Google, and tons of great restaurants and nightlife nearby), and becoming more of a residential campus compared to a commuter campus leveraging that now-hot location (although it still has a bit farther to go there).
While the in-state tuition rates at Illinois public universities aren’t that great compared to other states, they are still going to come out less expensive than out-of-state options for the vast majority of students (despite popular perception to the contrary). As a result, UIC is starting to solidify itself as the #2 public university option in the state.