Some of the Less/Sass maintainers are also active parts of the communities that write the specs and I guess when the idea of nesting was fresh to these preprocessors they didn't feel compelled to make it a core part of css with how widely the preprocessors were getting adopted. While I don't doubt our computers 10+ years ago could parse through nested styles it's probably really trivial now and thus being implemented into core.
As far as the speed of which things get implemented, I often feel there is a lack of understanding where design and development overlaps. Most devs are just devs in the sense they solve algorithmic/browser rendering problems, and don't really dabble enough with markup/styling languages like HTML and CSS to hit the pain points of using them. And the people that deal with markup/styling do not know the proper channels to rally behind some of the ideas that would make their lives easier. Unfortunately, there are less individuals that are proficient at markup, styling, and programming (not just in the sense of coding in JS/TS, but understanding the internals of the browser and programming for it), and the unicorns that do understand are often snatched up by Google/Microsoft/etc. which often have their own agendas. I feel there's just a lot more niche backdrop knowledge that is required to get the ball rolling. These are just my feelings on this matter, I could be wrong.
As far as the speed of which things get implemented, I often feel there is a lack of understanding where design and development overlaps. Most devs are just devs in the sense they solve algorithmic/browser rendering problems, and don't really dabble enough with markup/styling languages like HTML and CSS to hit the pain points of using them. And the people that deal with markup/styling do not know the proper channels to rally behind some of the ideas that would make their lives easier. Unfortunately, there are less individuals that are proficient at markup, styling, and programming (not just in the sense of coding in JS/TS, but understanding the internals of the browser and programming for it), and the unicorns that do understand are often snatched up by Google/Microsoft/etc. which often have their own agendas. I feel there's just a lot more niche backdrop knowledge that is required to get the ball rolling. These are just my feelings on this matter, I could be wrong.