how do you clock back something on google maps?
good question. i can only give you the answer i was fed in history class, i haven't bothered researching it and confriming it independently.
first of all sofia has been a city for a couple of thousand years but i assume you're asking how it became a major city.
well you see, when the russo-turkish war of 1877-1878 happened and russia won, we all sort of thought we'd get all the territories where bulgarians were the majority in the peace agreement. but the other great powers intervened because they didn't want russia to have a big balkan satelite and we got a pittance - the red part of the map is what we were given.
traditionally, our capital is veliko tarnovo - which loosely translates into something like great throne town. but it was determined sofia was closest to the geographical center of our struggle and the regions we had to forth to reabsorb so we needed sofia to be the place where we coordinated our efforts for unification with the other lost territories. so that's why sofia was made the capital. and when a town is made the capital, labor, resources and investments naturally flow towards it.