video conferencing...
Video conferencing generally is harder to set up (both ends require specialised equipment,, often in a special room, which needs booking so requires more paperwork). Audio conferences on the other hand can be joined by multiple people, each with nothing more unusual than the phone on their desk, or their mobile - booking a room with a conference phone is optional for the user if required.
In addition, the latencies on video calls can make them as painful to follow as audio conferences. TBH I'm surprised you see such a takeup of video conferencing in your survey. The question which apparently wasn't asked was 'what proportion of your conference calls are audio, and what proportion video'. I'd be surprised if this suggested anywhere near parity between audio and video.