PNP/NPN - Why do we not just use one of the two types?
There have been times when transistors with specific characteristics were feasible in NPN, or PNP, but not vice-versa. 

Two examples : 

The 2N3055 power transistor was NPN, at a time when PNP transistors of the same power were simply not available (115W I think, later pushed to 150W). 

That led to the "output triple" - a combination of 3 transistors that could replace either NPN or PNP output transistors, with the power handled in either case by the 2N3055. This was used in the famous Quad 303 amplifier of the early 1970s.

Another example : the BC214 PNP transistor could achieve lower noise in a microphone amplifier or audio input stage than contemporary NPN transistors.

With further developments in process technology, these considerations are less important than they were.