[smoldesu]

One important thing to remember about esolangs like this is that theory != implementation. Just because Brainfuck "theoretically" exists as a language doesn't automatically solve issues like memory management or extensibility. Sure, maybe it is more ergonomic to write some functions in K. But, as with most code-golfing languages, the goal isn't to build a better language, it's to build a faster one.

Maybe K can run those programs faster than Lisp/C/$FAV_LANG, but it's ultimately up to a much smarter programmer to implement that bit.

[wging]

K is not an esolang and was not designed for golf; it has real users. https://en.wikipedia.org/wiki/K_(programming_language)

Also, the primary goal is not always speed... runtime speed or speed of development. There are other things we trade off for all the time.

[the_af]

It should be noted that, according to Wikipedia, the K language isn't a toy language like Brainfuck but an actual language successfully used for a suite of financial software. It was developed for this purpose, not for code golf.

[TeMPOraL]

K language and APL family languages are serious business, though. And they get a head start on "extensibility" and other nice practical properties - notice how all those single-character operations are inherently and extremely composable.