I've mentioned this previously. Without government-mandated standards, implementation could take years. We apply this approach to numerous areas; why should IP be an exception?
As far as I know, the US federal government does have a mandate that agencies be ipv6-only by end of 2025. Systems that are not converted by then require justification for why they cannot do so along with a replacement plan. See https://www.whitehouse.gov/wp-content/uploads/2020/11/M-21-0...