....Switching systems is a massive undertaking, both from a time and energy standpoint, and a financial resources one as well. Fully switching brands must be a well thought out decision, and sometimes it simply isn't worth it - even when new product releases are inferior to the competition's.
sure -- and the sales figures (that show the clear Canon advantage) may be in fact due to the high installed based. the implication is that no one (with a lick of sense) really compares body-to-body they compare system-to-system. So Canon sells more bodies becasue their strategy is working.
I don't know that their current strategy is working but at a key point (lets say when the 20D/5D/5DII were introduced), they were able to capture a lot of photographers that were coming into digital photography or that were upgrading from point-and-shoot. Being on top of the market early let them establish a good installed base that will stick with the same vendor as time goes by because people can just buy a new body.
I think dilbert nailed it on the head.