Most people are going to use whatever is easiest and/or most popular. The same forces that make it hard to get all your friends onto Signal would also make it hard for violent people to use something new.

https://en.wikipedia.org/wiki/Nudge_%28book%29

(Not that I think the Signal team would ever go for any kind of weakened security, unless it was an absolutely final last resort.)

Because if these measures were taken, it would become common knowledge, and criminals would just switch to what's safer for their coordination. It may cast suspicion on anyone using these services, but that's not really enough.