Good question catmando,
The cost of extracting the oil was not cost effective for a long time, in the last while with the price of oil going up substantially and new steam technology used for separating the oil from dirt (simple version) has made the process worth pursuing and very profitable.
Currently the reserves are estimated at around 174 Billion barrels, making it the second largest reserve in the world next to Saudi Arabia.
The Province of Alberta has no sales tax, and each citizen gets an annual dividend cheque.
http://en.wikipedia.org/wiki/Athabasca_Tar_Sands
One can summarize that there is no shortage of oil, only a lack of "cheap oil" to power our Yachts