Well, first, his state of Illinois has an unemployment rate of 8.7 percent, the fourth worst in the nation. Second, Illinois is a fairly blue state. With polling in several Republican-held House districts around the country showing an unemployment insurance extension to be popular among Democrats, independents, and even in some cases Republicans, Kirk wouldn't have to be some kind of profile in courage to vote for the bill. Third, Kirk has on occasion been willing to take a bipartisan vote; for instance, he was one of seven Republicans voting yes on a procedural vote for the Employment Non-Discrimination Act.
But maybe Kirk is just sincerely and personally opposed to a safety net for those struggling financially—a standard, callous Republican. Except that when Kirk returned to the Senate after recovering from a stroke, he said he would be focusing on Medicaid, having realized that his recovery wouldn't have been possible on the number of rehabilitation sessions covered by Medicaid. Apparently that lesson in compassion was either very specific to Medicaid or very short-lived.
So—because 80,000 of his constituents have already lost benefits and 36,000 more will do so in the next few months, because there just aren't jobs in his state for all those people, because it wouldn't require a lot of political courage, and because he has in the recent past claimed to have learned about compassion and the importance of the safety net—Mark Kirk's vote against unemployment aid was especially bad. Even in the context of congressional Republicans.