Remember you try to hire a ${coder, admin, } not the next tv-news-presenter, beeing on screen is not a mandatory needed skill in most jobs.
By asking for something, that makes people uncomfortable, you will exclude a lot of likely brilliant candidates.
People who refuse to do video interviews may be for example: - people who value privacy, not only their own, but most likely yours too - people who feel very uncomfortable beeing watched by strangers and who think or even know that they will perform significant worse than in an audio-only interviewsituation - people who simply don't own a camera - people who use textonly computers offjob - poeple who have experienced that your 'standard'-videochat-app may not work, maybe because they use linux, bsd, os/2 or nonstandard operatingsystems - people who don't have broadband internet, yes there are still people like that - people who pay for every bit send, and yes having a not so cheap phone/internet contract is still common in some areas - people who feel uncomfortable to let strangers in their bedroom, even virtualy - people who have disabilities or cosmetic issues that they fear may distract you - people who have disabilities where moving and out-of-sync pictures distract them - people who tend to refuse unreasonable requests and who therefor regard you as unqualified to be their next employeer - ...
All of them have good reasons not wanting video interviews.
You, as an employer, may miss your best fit.