There aren't any rules (as there have been in some sports) so the women are competing in next to nothing through choice.
Is it comfortable? They all have a amazing bodies, but do they all like being on TV dressed this way? Is it performance? If so, why don't the men also do it?
Is there pressure from sponsors either directly or because the most 'saleable'get more sponsorship or advertising contracts?
I'm genuinely curious as to why they make these choices. I run and I wear less in competition than I do in training but wouldn't find high cut knickers comfortable to run in.