For me, it's because my job won't cover the pay that's lost because I have to go sit in on something that I'm sure homeless people or unemployed folks would be better suited for - ya know.. since they have the time and all to do something such as that. Plus, they'd get a crappy monetary check in the mail for doing so!
But seriously, I've been called in for jury duty once in my entire life, and it was a complete waste of time and a cut out of my pay I won't be able to get back. Nine days worth of pay that I lost ... and that can hurt a person when you've got lots of bills and student loans to pay. :/