He was caught up in the crazy Monaco youngster hype.
Jamdav said it pretty well in the EPL topic that, I think many people (Chelsea) were expecting a possible Yaya Toure style Midfielder but going by his performances this season he's nowhere near the same style as Yaya is.
Bakayoko was all right in his Monaco stint last season, but that team was stacked with young talents like Fabinho,Lemar,Bernardo Silva,Mbappe, Mendy etc and people were going crazy about that team and to be fair those guys were quality.
With Bakayoko there was a lot of people saying including Monaco fans saying he was easily one of the weaker talents in that group but had some room to grow as he is young. Fast Forward to his stint with Chelsea and it's been a disaster for him.
He's shown to be so poor on the ball and tactically inept. Can he improve? Certainly but I very doubt he would ever be that Yaya Toure style Midfielder some Chelsea fans were expecting.