Who can say? The mind of a motor manufacturer is unknowable.
But if I had to guess, Id say it’s some combination of:
- it’s technically weaker. Weak enough to be a problem for our applications? Who knows. But technically weaker.
- it costs more. It’s two operations instead of one, and you have to make sure that the two features are clocked 180° from each other, which takes extra care on a part which otherwise has no concern for how the various features are clocked. Not a big deal for experienced machinists, but slightly harder than not doing it.
- not all d-shaft axles/gears/pulleys are the same. So if the goal of the b-shaft is to be universal, it’s gonna fail at that.
None of those are particularly strong reasons, but people don’t generally need strong reasons to NOT do something lol