The two main culprits for the shortened season are the Osbournes: Reloaded special on March 31st, and the High School Musical/Bring It On ripoff called Glee, which "previews" May 19th, following the American Idol season finale. This leaves room for only six new Fringe episodes before the end of May 'Sweeps'.
It should be noted that Fringe has still not been renewed for a second season yet, but Fox Entertainment president Kevin Reilly did have some nice things to say when speaking to The Hollywood Reporter:
THR: What is the future of RemoteFreeTV?
REILLY: The jury's still out. We've gotten outstanding viewer feedback. I think the year hasn't helped. Fortunately, "Fringe" was a keeper. Can you sell the real estate for free? If we don't do it on a full series, we might use it for a special episode or stunt something. So we'll keep it in the mix in some fashion.
THR: There's a perception that "Lie to Me" and "Fringe" have been dependent on being paired with strong lead-ins. Is that fair? And even if it is fair, does it matter?
REILLY: No, it's not fair, and no, it doesn't matter. That's good television. "Fringe" has established a really dedicated audience, and "Lie to Me" is also very charming. Look across the board at 10 o'clock. Look at Thursday night, with the "Grey's Anatomy" lead-in and the "CSI" lead-in. Nobody forces anybody to watch a show even with a lead-in.