What do you have, some old obsolete parts?http://en.wikipedia.org/wiki/NPN_transistor#NPN
See the sections "Active-mode NPN transistors in circuits"
Basically saying that the base voltage must be higher than the emitter voltage for current to start flowing; when the emitter is grounded, the base only needs to be ~0.7V above ground to start turning the transistor on. When the emitter is sourcing current into a load, then the base voltage has to be 0.7V Plus the voltage across the load. If you want high voltage across the load, then the base voltage has to be higher. Much easier (efficient) to have a lower base voltage and sink current thru a load.
Similarly for PNP - much easier to pull the base lower than its collector to let current flow from collector to emitter into a load.