Some people from group A enjoy running and can run fast, therefore join marathons and are good at it.
Some people from group B enjoy playing table tennis and have great reflexes, therefore join table tennis tournaments and are good at it.
Some other people realise that marathons have way more A people than B people, and since marathons are prestigious they force equal ratio of A/B people in marathons, effectively reducing the overall quality of marathons and keeping A people (who would thrive) out of them.
Everyone is unhappy, except the ones who forced the outcome and the ones who got into the prestigious sport without deserving it.