Well there are also some other ways to see the thing:
1-As some usually say it may have been to hype kaido at the beggining of the arc featwise after been have hyped verbally as wsc in chapter 795.
Tbh i don't really beleive this reason.
2-luffy was maybe not yet yc1 level and his fight with katakuri maynot reflect yc1 powerlevel as that fight was hugely PIS as many say.
3-luffy may have fought the weakest yc1 aka katakuri and gained his fs during the fight.But fs can only take you so far against top tiers(if you don't have the necessary firepower) and without advanced coa,you can't do shit to them.
Let's say king has a good version of barrier and penetration haki and luffy fought him and defeated him before wano,he would've stood a much better chance against kaido in kuri than what he displayed.
4-Maybe oda didn't know what he was doing and just decided to retcon his powerlevels after marineford(Probably the most stupid reason,but who knows).