So, I did ask, and here are the deets according to Fender:
"The simple answer is the 115 cabinet was designed to match the footprint and complement the 410 with both supporting the Rumble head models. The 210 and 112 extension cabs with footprints matching the combos launched later to support both the combos and the heads, and having unique speaker complements for our cabinet offerings was preferred rather than 2 (size) versions with a 1x15."
Thank you, very much, TomBowlus! I'm sure everyone here appreciates getting the straight story direct from the manufacturer, even if the answer is not what some would want to hear.
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