If the voice masking wasn't well done, couldn't you end up with an uncanny valley sort of situation with respect to how the applicants sounded? I can imagine a scenario in which the voice sounds "wrong" at a gut level, and that makes some interviewers uncomfortable.
And, overall, what do you think the tech community would be most open to? A person that sounds like a woman, but comes off as a bit of a tomboy (on account of actually being a man), or a man that seems oddly feminine or "weak", "fuzzy" or whatever attributes you would assign to socially acceptable female behavior? And how did men that had their voice masked into female fare, compared to non-masked men?