Because brittle really is a second name for Flexi disc. They have shallower groove which means limited dynamic range, higher hiss/surface noise/distortion, plus because they are so light they often don't sit flat on a turntable so welcome speed variations aka "wow and flutter" and frequent skipping. You could listen 500-1000 times to normal vinyl before any audible degradation and the same may happen to Flexi disc after 5 or 10 times.