I'm willing to be corrected on this, but it's primarily historical. The dividing line was developed in the 1960s, so that everything west of the Ontario-Quebec boundary was to be supplied by western Canada and the area east of the Ottawa River was to be supplied from overseas. During times of little or no political volatility, when the energy was available and affordable, it made sense.
On what has happened over time, as has been pointed out, with the OPEC crisis, line 9 was installed with the intention of overcoming this reliance on countries outside of Canada. Since then, line 9 has been reversed. One of the underlying assumptions is that we don't need to reverse it again, but we do need to reverse it now to meet not necessarily our energy security, but the energy security of other countries. From my understanding, it was a historical decision to take that path and supply eastern Canada with overseas crude.