Why does a fluorescent light burn out?

The two main reasons I know of usually deal with the balast that powers the bulb. As long as the powder and the mercury (or whatever it is now) are in the tube then it should last.

One thing I have also heard is in reference to the ground plane. I remember being told once that, the old trick to get a fluorescent to work was to drag your hand along the tube length, was an attempt to resestablish the ground plane. I can't help you with that though. Maybe an EE might chime in.

Check out here too: http://home.howstuffworks.com/fluorescent-lamp4.htm

 

Let's not take into account the balast aging, I think one reason for a fluorescent not working is its filaments. When the filaments get weaker, they can not supply enough electron flux for the normal operation.