Because it has a 'bulb' like shape! Not sure what you are actually asking Awesome... You could equally ask why is a 'pig' called a 'pig'. This is the RHS definition of a 'bulb' http://www.rhs.org.uk/Plants/RHS-Publications/pdfs/BulbSpread

Because it is bulbous.

That is - "Fat, round, or bulging."

Hence bulb-shaped: Like a 'bulb' or indeed a lightbulb.

Which word came first though, Fractal?

The word comes from the Greek 'bolbos' which means onion.
Ask the ancient Greeks why they used that word!

lol Owdb!

Good question, Bulbaholic...
The answer is chicken. Or is it egg?
One of those! :)

;-)