In general, despite great promise and utility (IMHO), curated bookmarking of this sort never really took off. It's always been around, and still is. Why? some guesses:
Not that many people are data-oriented enough to want to use such a service. Same reason RSS is relatively unpopular.
Delicious and related services never generated the sort of social proof / payoff that drove other services. E.g. desire for friends / followers / karma were essential to FB / twitter / HN (since we are here!). Delicious was high on the personal utility, low on the social reward.
I'm not sure this is that related to Delicious' functionality, but might just be a cultural / community function, that never really developed in this case. It's not tech that makes HN awesome, it's people. But given that Digg / Slashdot faded, this may not be as popular a category. Reddit is huge now... but it's more of a forum vs. link sharing.
And these services are so easy to create, who would pay for them? In the absences of hype / popularity, not much of a business there. I switched to raindrop, which is great. But others were close.
It's very sad to me. I think that the ability to socially curate the web, and to vote up/down on such resources, is actually really valuable to the long-term health of the internet.
Maybe we're just not ready yet...