Why does paint on metal crack usually?

There are many different types of paint. Also, metal is a non-porous surface, mostly - you can increase its porosity (or absorption, ability to hold paint) by sanding or otherwise roughing up the surface so the paint "grabs" better and has less probability of cracking. Temperature and other outside variables can also cause your paint to crack, depending on the type of paint, how it was applied, etcetera. Go get some paint that is specifically designed for metal, rough the surface up with some sandpaper or steel wool (rough stuff, not fine, you won't get anywhere with fine grit/wool)

• make sure the surface is clean, paint it with some proper primer

(or at least wipe it down with some alcohol) and then paint it - you should see nice results, given the metal in question isn't going to be flexing, bending, or taking excessive pressure - in that case, then really, there's nothing you can do - just sand it down and keep it bare metal.