A couple days ago I needed the correct word for a female aviator, which I figured was aviatress. A dictionary.com search provided aviatress, aviatrice and aviatrix as acceptable choices. Coincidentally, I had just read this EL&U post about the origin of the -trix suffix, and so I explored other Latin-based agent nouns. I found that legislatress exists along side legislatrix as does benefactress with benefactrix and orator with oratress. Not so with dominatrix, though. It seems to have maintained only the -trix suffix.
Dictionary.com has this to say about the -trix suffix in modern use:
Most nouns in -trix have dropped from general use, so that terms like aviatrix, benefactrix, legislatrix, oratrix, and proprietrix occur rarely or not at all in present-day English.
Dominatrix has maintained, and most people would only ever use it in the realms of sado-masochism and sexual role play.
My question is, why has the -trix suffix fallen off and been replaced by the more sibilant "-tress"? And, have we avoided the -trix suffix to avoid any unintended association with the sado-masochistic sense of dominatrix?