In an interview with Mynavi News, Canon executive Kengo Iezuka says that their recently released 600mm f/11 and 800mm f/11 lenses would’ve been impossible to produce for DSLRs. They had planned to attempt these for DLSRs, but due to AF limitations, it couldn’t be done. Their creation only became possible due to the design and capabilities of their new RF mount mirrorless cameras.
Mr Iezuka is one of the executives responsible for the planning of lenses at Canon and also hinted that more such lenses might be on the way, depending on the success of these two. In a world where people typically want primes to be as fast as possible, it could be a tough sell – of course, these do have that budget advantage.