Q:

Why does an airspeed indicator not simply read TAS?

If the airspeed indicator has inputs regarding air density (static port) and temperature. Why isn't Indicated airspeed from the indicator simply the same as True airspeed

A:

Well, indicated airspeed is the most important speed a pilot needs to know, so it makes sense to have that in the most obvious position. 
IAS can be thought of as the pressure that the wings are experiencing - therefore all of the stall, manoeuvring, flap extension, and landing speeds etc are based off indicated airspeed. 
True airspeed is really only important for flight planning and navigation. Given that more and more aircraft have GPS nowadays to give the groundspeed, the need for the pilot to know the TAS in flight is further reduced. 

