plot, the same reason BM didn't fight Luffy but Katakuri did.
Understimating Luffy too, saturne thinks kizaru and Lucci are enough but when things get out of hand he will forced to fight.
there is too much on the field to not do so, the motherflame, Punk Records, york and most above all killing yonko luffy who will be left weakened after fightning kizaru.
Saturne won't miss that opportunity.
Once why the reverie flashback with sabo it was to prepare the reader for that moment so they dont say there was no buildup.
In this arc, admirals are being written off as main threats and enter Gorosei for final war.