The other main reason is that upgrading the tooling for sensor fabrication is super super expensive, and Canon has historically used the high-volume entry level

The other main reason is that upgrading the tooling for sensor fabrication is super super expensive, and Canon has historically used the high-volume entry level camera market to subsidize this, while sony uses their smarphone business to subsidize it. Canons entry level camera market is tanking just like everyone elses which probably made them weary of such investments in the past few years (till Sony came and stole their lunch). It seems like they've decided to go for the high end high-profit segment. I'm super interested to see what kind of sensor there is in the R5. They're probably not at A9 read-speeds quite yet, but it will be interesting to see how close they've gotten.