M series chips are ridiculously massive, as Apple apparently does not want to transition to chiplets, so they can’t easily compose CPU. Thus refining the process and improving yields on the smaller parts probably makes sense.

As an other example the current ultra part is the M3, and it was released early 2025, after even the M4 Pro/Max, and a good 18 months after the M3 was unveiled. We might not see an M4 Ultra until 2027.