Basically Rockshox made the only really good adjustable fork, uturn, but abandoned it because turning a dial a couple of times was too much like hard work.
TALAS still exists as does Rockshox dual position. Neither’s a truly adjustable length fork though, they’re just impaired climbing modes- with uturn you got a fork that genuinely worked properly at all settings.
The other thing is, they tend to have drawbacks. I’ve not used the latest TALAS but it used to be noticably worse than the equivalent float, especially the old 3-position. Dual position’s only recently gained the ability to use air spacers (which IMO is pretty essential on the Pike). And there’s frequently reliability and cost issues too.