what makes kdb so special and why isn't there an open/libre alternative?

k variants:

kona (C, interpreter): https://github.com/kevinlawler/kona

klong (C, interpreter): http://t3x.org/klong/

kuc (C++, JITted): http://althenia.net/kuc

oK (JS, interpreter): https://github.com/JohnEarnest/ok (see also iKe by John Earnest)

cousins:

J (C, interpreter) http://jsoftware.com/

A+ (C, interpreter, unmaintained): http://www.aplusdev.org/index.html

Gnu APL (C, interpreter): https://www.gnu.org/software/apl/

apl.js (JS, interpreter): https://github.com/ngn/apl

There's also NARS2000 and a few other APL interpreters

related:

Numpy and R provide similar functionality, albeit with more verbose (and less fluent) composability. They are usually slower.