Are these competitions behind a screen? I'm only somewhat familiar with the Can Cliburn competition, and it seemed as though it was in the open on a stage.
If they aren't blind then I'd expect gender bias is a more obvious explanation (obvious because the effect that blind auditions have had in the gender make up of orchestras is obvious and dramatic)
I went looking for other data that might let us make conclusions about the situation. Here's what I found:
Cayea, Danelle, and Ralph A. Manchester. "Instrument-specific rates of upper-extremity injuries in music students." Organ 26.362 (1998): 11-14.
The high-injury-rate instruments (12.0 to 18.0)
included the piano, guitar, and harp. Women had
a higher overall injury rate thanmen (8.9 vs 5.9).
Small sample sizes for some instruments (e.g. only ~20 female trumpet players) make me doubt the solidity of the results, but it looks like bigger instruments are vastly more dangerous for women to play. The piano and organ are both pretty bad, with about 10% of women reporting injuries compared to about 7.5% of men. The double bass has the largest difference in the study, with 17% women reporting injuries compared to 8% of men. I have no idea how harps compare because there were exactly zero male harpists in the sample, but female harp was tied with female double bass as highest-injury sampled instrument at 17%.
Note that, as the studied population was "the students at a single institution's music major", an RSI injury this early might indicate the beginning of a chronic, career-limiting problem.
I can't find any competitions for the double bass that I might be able to use to confirm the correlation between injury rate and under-representation in high-level competition.
TL;DR I find it quite plausible that some instruments are objectively difficult for women to play as indicated by a higher level of upper-body injury.
If they aren't blind then I'd expect gender bias is a more obvious explanation (obvious because the effect that blind auditions have had in the gender make up of orchestras is obvious and dramatic)