Mark Kirk was a really, really good candidate. In 2008, a blue tidal wave year, Mark won reelection in IL-10 by 6 points as favorite son Obama carried it by 22. He was a very moderate Republican, and that's the only kind of Republican who has a chance of winning statewide in IL. He was to the left of Collins/Murkowski, and probably even Manchin.
He only lost as badly as he did in 2016 because his stroke completely messed him up. It would have been better to have him retire and let the GOP run Adam Kinzinger or Bob Dold.
Giannoulias also had some ethical issues, and Illinois Democrats were dragged down that year by the stink of Blagojevich.
The case for Mark Kirk being a good candidate would be better if he was able to gain with the same constituencies that Trump did from Obama voters while holding his numbers in the Chicago area. Instead, he lost by nearly the same margin as Trump, trading gains in the Collar Counties for a large underperformance in southern and central Illinois.