I think it would make some sense, if Moiraine was the Blues' Head and thus also a political power to support Siuan, especially with the foremost of all Warders, Lan, at her side. Moiraine is certainly is the most capable and powerful of the Blues. I expect that especially the Blue Ajah is such wise to consider that it makes more sense to have a capable person as Head than necessarily an older AS. Usually people suggest that Anaiya was the Head of the Blue Ajah the whole time. IMO, she only became Head after Moiraine's "death". In Moiraine's conversation with Anaiya in TGH, ch.4, I got the feeling that Moiraine actually ranks higher than Anaiya, because Moiraine never states in her PoV that Anaiya is the Head (just a "friend" and likely a Sitter), but Anaiya tells Moiraine that the Blues miss her, which makes sense if Moiraine is the Head. It's also never stated how often and how long Moiraine was away from the WT in the last years, just by Anaiya that "this time" Moiraine has "been been gone from Tar Valon too long. Much too long. Tar Valon misses you. Your sisters miss you. And you are needed in the White Tower." But even the Red Ajah Head Galina leaves the WT for her mission to capture Rand. It could also explain better, how Moiraine got her angreal, created her good spy network, etc.