There seems to this general consensus over here that Ayaka was going solo because otherwise her Mistplitter banner would be «too good». Here my issues with this:

• Character banners is their main source of revenue. To let the less popular banner dictate their cash cow makes no sense.
• What if they ran Yoimiya with Ayaka, and we got Thundering Pulse/Mistsplitter. A fantastic banner. So what?
• It’s not like people pull the weapon banner because they want either Thundering OR Mistsplitter, they all want BOTH or ONE of them. No one is going «I just want Thundering Pulse og Mistsplitter, I don’t care which». This means people will pull until they get what they want.
• Thus, the only thing you do by pairing Mistsplitter with say, Memory of Dust instead of Thundering Pulse, is to make the worst case much worse for people. This will STOP people from pulling, since they cannot afford or risk sitting there with two Memory of Dusts.
• If you ran with Thundering Pulse instead of Memory of Dusts, more people will pull since the worst outcome isn’t THAT bad. No one would stop when they get Thundering, they would just keep pulling for Mistsplitter and not be that unhappy.
• The only meaningful argument is that the few people that want both Thundering Pulse AND Mistsplitter will now pull on two banners instead of one, but I am willing to bet my ass more people will rather SKIP both of them if they are paired with trash. At least from my own experience and what I see here on reddit, the majority pulls on weapon banner only when the worst case isn’t that bad.

Hoyo obviously has reasons for doing solo Ayaka, but I cannot fathom that they would be doing it just to sabotage their Mistsplitter banner.