There was an 80s TOS novel, I cannot recall the title, that offered an explanation.
Something along the lines of a starship being on the frontier, where no man has gone before, with no resupply port or repair base in range. The ship and crew were on their own and had to be able to dismantle, repair, refurbish, etc with only their own hands out in the field, as it were.
Slick interfaces and high tech was eschewed in favor of easily swappable, reliable, rugged, basic controls. Much as field gear today is military-rugged versus a fragile touch screen iProduct for home consumer use.
Does that sufficiently explain TOS retro tech?