I can't think of a single good reason for it to be built planetside other than to have Kirk sneak in to look at it and it seems to me he could just as easily have taken a shuttle trip and snuck into a shipyard to view the construction.
I can. Access to labor.
Likely there are more warm bodies on the planet, willing to work for less than warm bodies living on a space station.
Say what you will about the ease of relocation and the effectiveness of the transporter allowing you to move people, but even logistically it would be better to have that labor pool live and work in its natural environment.