They have other things to do. The modern Senate has more staff, deals with more interest groups. There's more legislation. More appropriations. The modern senator spends 1 percent of his or her time on the Senate floor. They have to take pictures with constituents. They have to fundraise and meet with constituency groups and lobbyists and deal with staff. To actually have a live filibuster would mean they have to give up all the other business.
And as individuals, they have other things to do. Air travel has opened up. In 2009, if you are the senator from Montana, it's perfectly reasonable for you to go home on the weekend and campaign for reelection. That wasn't possible in 1940. You came to Washington to do your work and you stayed until it was done. Now air travel has made it possible for you to fly away for the weekend. That makes your time more valuable.
Jonathan Bernstein adds an additional wrinkle, the modern media:
In the old days, Senators engaged in a filibuster would read recipes or otherwise stray off topic. No need for that now! Not only do Senators have large staffs who could produce content, but there's a whole big internet available. If I were advising the GOP in that situation, I'd tell them to let conservative bloggers know that they can have their big chance for immortality: post something good, and a Republican Senator will read it on the floor of the Senate. Doesn't even have to be about health care! Excellent way to rev up the conservative blogosphere, no? Meanwhile, by forcing Republicans to perform a "real" filibuster, Democrats would transform a 24 hour network that millions of Americans get in their homes into a 24 hour Republican propaganda outlet. How is that possibly good for the Democrats? Granted, CNN coverage of the Big Filibuster would invite comments from both sides, but even CNN would probably go to the Senate floor once in a while, and each time it would be a Republican talking.