Probably because Chelsea had secured the deal with Potter to be the next manager. If only there wasnt anyone available and better than Tuchel then I doubt that he will be sacked. They probably think that Potter had the potential and he accepted the offer so yeah they sack him right on the spot after losing the game but its just brutal at some point
The appointment of Graham Potter as Chelsea new coach is none other than because he is considered capable of changing the position of players and being able to adjust the tactics he plays. apart from that we can see he is able to lead Brighton to perform better at the start of this season. No wonder the chelsea boss directly interacted with Potter in submitting an offer as the new coach of chelsea and he accepted without any pressure. I read in some news that was published in a number of media Tuchel dismissal because there was disharmony between Boehly and Tuchel so that it could have an impact on the dismissal decision.