Billllyuns and billllyuns. Light bulbs are part of the interstellar "goo" that pervades our universe; they are star stuff.
or
Use a sea saw.
or
Because he wanted to draw the curtains!
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
He wanted to make a clean getaway!
or
Because he wanted sweet dreams.
or