So what happened was that many of these packages existed pre-1.0 when there was no stability system. Nobody knew which features would be stabilized (if you go further back, the fact that things would be stabilized in such a way wasn't clear either).

So we all just used whatever feature we liked. Many of these unstable APIs or features had stable counterparts, but the choice to use an API/feature was made pre-stability, so nobody had any way of knowing. Which meant that a lot of unnecessary unstable API usage persisted after 1.0. Some crates made a transition. some were slower (I think hyper was pretty quick to stabilize). Servo, for example, didn't have any issue with using nightlies for various reasons so we kept using unstable APIs (I later audited and removed low-hanging extraneous unstable API usage. Most of our out-of-tree crates are stable too).

There have been efforts both by maintainers and by individual community members to bring crates to stable, and I think at the moment most useful non-plugin crates work fine on stable. Let me know (@Manishearth on twitter) if you find a crate that you need that doesn't work on stable!