My hunches:
A. Every caucus-goer in Iowa, and most voters in New Hampshire, know that:
Predictions of "Biden's un-electability" were the main assumption behind Buttigieg & 10 other centrists entering the race,
Durability of countervailing confidence in "Biden's electability" has been the only thing keeping Biden afloat in opinion polls,
Predictions of "Bernie's un-electability" were the main assumption behind Warren entering the race,
B. Last-minute emphasis on "Electability comparisons undermines unity of Democrats":
Will insult many in Iowa and New Hampshire who see these comparisons as part of their "early state" job description -- including many who previously leaned towards non-Bernie candidates precisely because of electability, and
Will annoy and perhaps help turn out many independents who will perceive (correctly ) that "unity among Democrats" implies "double down on excluding independents from influence before the final lesser-evil choice."