Different Chip SKUs are often a TON of work. By trying to release all of them at the same time, you'll have a chip pipeline where you need tons of work, all at the same time, all in the same stages of the process. By staggering them, you spread this work out across the year.

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.