Yamato: I'm sure you all noticed too, right? The incident with the “forest guy" was only resolved thanks to someone's powerful Haki. Kaidou may have fallen... But now we have all those guys coming after Wanokuni!! I cannot allow it!! How could I go on adventures while I'm worried about this country? I'm sure we both feel the same way!!"
I was right on why Yamato was not joining the crew.
GB attacking wano was all she needed to realize Momo needs help protecting it.