During very early days these DIY mech keyboards are all someone's hobby project, finding a low cost, easy to program microcontroller with USB HID support and enough GPIO pins for keyboard matrix is surprisingly hard. ATmega32u4 (and particularly Teensy/Arduino Pro Micro) is popular at that time, the two popular off shelf boards only have 18 GPIO pins usable, the absolutely maximum key count you can get is 9x9=81 keys, not even fitting a TKL, and that's assuming extremely weird routing, for a more sensible routing you wire a 6x12 keyboard matrix, that's 72 keys max, which is why most custom keyboard from these days are 60/67 keys.

After that it's basically a cultural thing.