Like in any gold rush getting gold at the newly discovered mine is pretty easy... as time passes the mining gets harder and results for physical gold are not as easily to come by.
Bit coins are mathematically designed in a similar fashion for the same purpose. the more that is mined the harder the next bitcoins are to be acquired.
This Pod Cast explains the answer to your question much more clearly:
"We’ve moved beyond the Mises textbook. We’re running in the open market." - Erik Voorhees