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.
Flexi discs were cool inserts for some home computer magazines; you'd dub the flexi disc to music cassette, and the noise and beeps were computer programs for your home computer.
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I remember our home stereo in the 1970s had a rubber non-slip-matt with concentric grooves in it, often flexi-discs were so flexi that that the needle would follow the groove on the other side instead of the one on the disc. They were almost unplayable on that unless you set the tonearm counter-weights to as light as possible.
That built-in obsolescence was often the point of flexi-discs as they were typically used as giveaways in magazines with the goal of promoting the artist - the faster it wears out, the sooner the consumer is likely to go purchase the real thing.