Good points, Engines.
To which I would add that the switch from 27 mm Mauser to 25 mm Gatling came at a point where LM was blithely and quite inaccurately convinced that they had weight to burn. Plus, as mentioned, the LCC issues of bringing a new caliber into the system. However it turns out that the 25 needs a new round to deliver A-G and A-A effects without using depleted uranium. Oh well, it's only the taxpayer's money.
Also, as Engines notes, tests of integrated fire/flight control in the 1980s showed higher gun lethality. The Sovs were on this earlier and consequently both cut down the number of rounds and reverted to a non-revolver cannon on the Su family (GSh-301 and 150 rounds, 2800 ft/sec MV). However, it was not fully recognized when the F-22 design was set.
Sticking a Gatling in or on a stealth aircraft is about the most expensive way to go - but that's where we are.