Part of the trouble is the terminals are made to grasp plastic insulation, as strain relief. With enamel instead, there's nothing to grab, and stress is applied directly to the crimp section, which is already stressed from having been crimped.
Even with [grasping from the end prongs], it's solid wire without strain relief, nothing to increase its radius of curvature when bending force is applied. So it fatigues easily.
You may've also nicked it while stripping off the enamel, depending on exactly how you did it and all. But since it broke at the crimp, that's probably fine, and it's simply as above.
Whereas, as intended, the double whammy of plastic insulation acting to pad the bend, plus being divided into strands, the bending radius per strand is much larger in relative terms, so the flex life can be massively higher.
Tim