> What are the major iSP's incentives for not peering?

In the US, each peering agreement is a private agreement (ie. a contract) between 2 parties, usually with roughly equal bandwidth traffic.

If one party doesn't like you, perceives you as a competitor, or you don't have similar bandwidth, then no deal.

Examples of typical rejections are Cogent, who compete on price, and Netflix, who need last-mile acess to consumers.

https://en.wikipedia.org/wiki/Peering

For Canada, specifically Bell, they both an eye ball network and a transit provider. Bell has no incentive to peer at local IXs and "give away" part of their service (the eyes, as that is the lucrative part) when they can sell on the fact they are a large national network AND you can get access to their end customers as well.

Edit: clarification on the part that is more valuable.