That’s odd. Are there standards in iOS Safari that macOS Safari doesn’t support, or that Firefox doesn’t support? It seems that if you have iOS in your user base, you couldn’t do Chrome only stuff anyway.
And no, it's not because there is a standard Firefox does not support. I have installed a user agent switcher add-on on Firefox and that has been sufficient to let me use them. Like back in the days of IE, web developers do not care about users of non-dominant browsers to bother testing on them, and prefer to block them from access instead of risking receiving the occasional bug report or complaint from someone on Firefox or Safari.
Personal experience—there are a raft of minor differences between Chrome and Firefox, just to pick two. For example, they parse SVG paths differently. The exact behavior of <audio> tags when starting and stopping them used to be different (don’t know if it’s fixed). There are a bunch of other minor things. I have many times discovered that my app doesn’t work in Firefox if I’ve been developing in Chrome, or vice versa.
If you write a lot of weird code, you can easily end up with broken features or a blank app and stack trace after switching browsers. Of course, 95% of the time it will work fine.
Some also have the luxury of a production environment that is separate from that environment.
On Safari I get extremely low FPS in general when drawing on layered canvases and sound is not working at all.