“There simply were no users asking for it and there were almost no developers interested or knowledgeable enough to work on it.”
ISRG paid for it, and curl shipped a Rust HTTP backend marked experimental for four years. It got about 95% of the way and nobody showed up for the rest. Rust users use hyper but don’t want to work on a C project, and curl users never asked for it. Memory safety is easy to demand and hard to staff.