Illinois is one of the more active states for this kind of promo conversion, largely because of how many licensed sportsbooks actually operate there and how the state structures its promotional rules. Here is what is actually relevant if you are working from Illinois specifically.
Illinois has real depth of licensed operators
Illinois legalized and launched mobile sports betting with a large number of licensed operators competing for customers, which means more welcome promotions are typically available to a new Illinois resident than in many other states. More licensed books means more one-time welcome offers, which is the single biggest factor in how much opportunity exists for anyone in a given state.