It absolutely is a question of "what does it cost the black Mage to do this?" You're asking for a Black Mage to specifically be in umbral ice and at the precise part of the minute both swift and triple are up, both cool downs which BLM absolutely uses on cool down for dps gains, to raise four people in a row. The Black Mage is either going to passively sac it's DPS "just in case," or it's not and people will most likely drop like flies while triple and swift are on cool down.

Summoner was often seen as the stronger caster than red mage throughout the latter's life due to greater mobility (ruin 2 was only 20 potency loss), vastly superior DPS (it surpassed or rivalled black Mage throughout StB and is apparently back up to par now), and the opportunity cost for it's raise was much lower (120 potency for the Lost gcd vs RDMs 310 flat potency at the time). As soon as bleeding edge ended RDM wasn't needed anymore, Summoner was that much stronger. Chain raise is a very, very niche thing and often by the time it's needed you're not clearing anyway; see Titan Savage week 1 where RDM's "key feature" was useless because a single death basically locked you out of a kill that run.