Because each time you bend it back and forth, you're stretching it past its elastic limit, which fractures it. Simply gets worse the more times you do it.
I think we'll let Rat have it . What I was looking for is when metal is bent, it's crystalline structure becomes more and more regular, eventually becoming regular enough to be brittle and fracture.
(AKA work hardening when it's done on purpose (but not to the breaking point) to make it keep its shape [better]. Also why you can pinch wire slightly with a cutter and then get a clean break.)
I knew I'd learnt it twice (In both Chem and Physics) but I couldn't quite recall the proper terms to use. Then I got lazy and forgot to check the topic later, or look up the proper terms.
Ratty, hurry up and post a new one, perhaps something in the field of Physics? I've got an infinite line of "Charge!" here for you if that'll help.