I dunno, they seem to do this sometimes, mix componentry on bikes like this. Again, not sure if it's to keep all partners happy. "Sorry, we've made an agreement with Bosch to supply the motors this year, but you can provide the groupset and brakes." Kinda thing. Who knows.I don't think its about keeping shimano happy, since the same bike that canyon resolved to use the EP8 on, comes with a full sram eletronic drivetrain. In fact that surprised me even more: why did they choose tho use the shimano motor and not take the advantage of full integrated drivetrain and shifting control?
The costs involved in servicing a motor should also be considered. There are fragile PCB, bulky bearings that are sealed and sprockets that need lube that may compromise the PCB, tinny sensors and I/O devices all housed tightly, in such a way that very few riders could DIY and even fewer mechanics are prone to work on.
Or maybe on all the new bikes they were making they needed, say 3000 motors and Shimano could only supply 1500 in the timescale required, so they had to split things. I kinda doubt it though, as these bikes are planned so far ahead.
Maybe they just know that Bosch is more popular or wanted to offer the Bosch features.
You mention that it's hard to service a motor because of the size, but Bosch and Brose manage... And they're nowhere near as compact as something like a watch or mobile phone, for example. It shouldn't be nigh impossible to replace bearings, especially since they're a wear item.