Eastern Canada refineries process light sweet crude, which is largely imported. Some Alberta refineries can process heavy sour crude, which is the majority of what Alberta produces. That crude can’t be processed in Eastern Canada. So there is no point shipping that oil across the country. Heavy sour gets processed in Alberta (only making diesel and road asphalt bitumen) Texas and Oklahoma.

As others have pointed out, we don't have heavy crude refining in most of Canada. The vast majority of Western Canadian crude isn't "Oil" in the common vernacular of the term, what we push through the pipes is diluted bitumen or "DilBit." DilBit is effectively raw bitumen sand that's been dissolved in some of the most caustic chemicals man has ever devised. It's effectively a liquid sandpaper that is highly volitile (tends to explode from time to time), abrasive (tends to scratch and wear down pipes and pumps), corosive (it's literally melting rock), and full of toxic impurities some of which are even mildly radioactive. Unlike the oil that comes from conventional wells, DilBit requires additional processing most Ontario refineries aren't really equipped to handle, and produces some waste material called petrochemical coke, or pet-coke that requires disposal after the fact. Intermediate upgraders that can convert DilBit into synthetic crude or SynCrude exist, but there are relatively few of those in Canada. It's more cost efficient to do the upgrading at the same refinery that's going to be processing it into finished product.

Most "Heavy Sour" (oil with a lot of impurities, some of which are toxic) refineries in the world are in the US, used for processing shale oil, Alberta oil and once upon a time and likely again in the future, Venezuelan oil. China and India also have a few Sour-Heavy refineries, but much smaller scale than the US.