May I ask what made it special? It does look awful "elegant" and well put together ...

when Cappuccino was written, Apple was still using the Objective-C + AppKit stack. It had been honing this stack for years, every WWDC brought useful and deliberate improvements. Back then macOS development was much more niche (and tech in general). They could take their time. MacOS releases happened every couple of years, not every year. Not the kind of outcome we have nowadays where changes are rushed because of demand.

Cappuccino improved upon javascript by using a different language (objective-j) that added many features js didn't have, and it used the same (mostly) well-honed api as AppKit. In comparison to other js UI frameworks of its time, it was lightyears ahead.