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?
Already "Where No Man Has Gone Before" illustrated the practical advantage of such simplicity: http://tos.trekcore.com/hd/albums/1x...eforehd536.jpg
Then there is the issue of durability. Have the ship encounter a couple of gamma bursts you'd wanna make sure your equipment is adequately shielded and still functions later. I believe the field equipment could be drowned, dropped, fried and micro-waved yet still function later. Try this with some of our "toys" (actually - don't!).
Considering all the failure or malfunctions of the inertial dampers, I'd think throwing a physical switch is a better way to ensure you actually pushed the right button in a critical situation.
And do only Humans, Vulcans and Andorians handle starship control or would you like to make controls compatible so even a Tellarite
(or a Gorn) could manage these?