There was Cappucino by ex-Apple employees, and actual Apple devs had SproutCore. So where did they go? Why some unknown libraries?

It's using Svelte, I wouldn't exactly call that unknown. Why maintain your own library when a third party one does exactly what you need?

Unsurprisingly there are many frameworks/initiatives that end up falling by the wayside over the years, e.g. MacRuby was being lined up to supersede Objective-C for app development at one point.