Best reason I can think of is that the calibre 321 is the movement that was on the moon. Not the derivative, not the descendant, but the actual movement.
Additionally, the long term corporate goal expressed by Omega President Urquardt circa 2012 was a transition to all in-house, all coaxial movements, for all but one: he explicitly said Omega is committed to maintaining one hand wound movement in perpetuity, and that's in the Speedmaster moonwatch.
If going through the overhead of maintaining a single mechanical hand wound movement for one purpose - the heritage unique to the Speedmaster, why not do it right, and go with the actual movement that was on the moon?
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