Like I said laws and cost. For example in the US everystate has different window tint laws. Here in the GCC every country has different window tint laws. Even here doesn't matter what car you have if it's under a company name you can't have it tinted.
And making electrochromic mirrors is much cheaper than making electrochromic glass.
Remember Barry use to work at a shop installing electrochromic tint and how damn expensive that was.
And outside of the middle East people really aren't too concerned it obsessed with tinted windows.
Over here it's necessary, a must have. Other places not so much. So maybe if there is a huge demand for it l, it might be offered as an option