Few factors, often discussed in the Air Canada forum
1. Much higher taxes on tickets in Canada (GST, NavCan, etc)
2. Air Canada's monopoly in Canada
3. Buffalo's perceived lower economic status as a market
4. Jetblue/Southwest/Air Tran all have a presence in Buffalo
5. YYZ being one of the most expensive airports in the world to land a plane
6. The limitation of BUF to short/medium haul domestic flights

You should make your flight no problem assuming the Greyhound is on time.
You can always take a taxi if need really be, its about $25 to the airport.