Illinois is a populous state. Even with the shift in population from the old industrial midwest to the sunbelt, Illinois still has many people. In terms of electoral votes, it is tied with Pennsylvania for fifth place, with 20 electoral votes. In the general election, Illinois is almost certainly a state that will vote Democratic, especially since it is the home state of Barack Obama. And even though the delegate allocation rules in the primary favor Republican-voting states, Illinois is still rich in delegates.
The early part of the primary race was a bit of a patchwork, but at this point the map can be filled in with only two colors. Mitt Romney winning in affluent, mostly suburban, communities, while Rick Santorum wins in rural and lower income areas. The only question at this point is exactly how fine the line is drawn. In both Michigan and Ohio, the line was drawn finely indeed. In Illinois, either because the demographics are slightly different (Chicago being a much larger city than either Detroit or Cleveland) or because the race has shifted, the answer came out a bit more clear cut. Mitt Romney won the state with 47% of the vote against Rick Santorum with 35%. The other two candidates scored under 10% each, and will not qualify for delegates.
If this was earlier in the race, I would say that not too much to the result. The state's demographics weren't favorable to Rick Santorum, he was heavily outspent, and Mitt Romney still couldn't pull out a majority. But it is getting late in the race, and while there are a number of large states that Rick Santorum has a good chance of winning, such as Texas and his homestate of Pennsylvania, they won't make up, either in delegates or in perception, because he has already lost too many big states.
To use a basketball metaphor that hopefully isn't overly folksy, before Illinois Santorum was like a team that was down by ten points at the half. Now, his position is more like a team that is down 15 points at the beginning of the final quarter. Even for Santorum to force the situation to go to a brokered convention would require a string of good luck on his part.
Of course, Illinois also raises some questions about Romney's performance. Even if we accept that he has finally become an "inevitable" nominee, we still have the fact that he couldn't clear a majority in a state that should be demographically and politically favorable to him, even after spending close to four million dollars, and against a candidate who is not particularly charismatic. Although it isn't unprecedented to have a candidate make it to their nomination the hard way, Mitt Romney seems unpopular with much of the Republican base, and even in Illinois, he is hardly the candidate by acclamation.
From here, the contests will be fewer and further between, and there doesn't look like there will be room for major surprises. If Romney does win the nomination, it will be hard to say when the turning point came. After covering the primary process for this long, I fear it will end not with a bang, but with a whimper.