The owners of Chelsea are not having any patience, they want results because they have spend too much money on the club but they are not seeing their money being used very well to better Chelsea as a football club. Pochettino would not have taken Chelsea to the level the owners of Chelsea want to see Chelsea playing at. There is no time to give the coach that chance to build up the team slowly, they want a fast response and for Chelsea to be playing with the top clubs. The sacking of pochettino is not going to be a regret by the board because he is not the right man for the job in spite of how well Chelsea were playing at the end of the season but by then it was already late because he did not start strong when the fans and everybody were expecting him to use his experience to make the players play good football.
Perhaps the point is not that Pochettino did a bad job, but that Chelsea believe that Maresca can do a better job. And they didn't want to miss the opportunity to bring him in as head coach because someone else might do it. I also think that Pochetino did well, by the end of the season Chelsea really looked better, but apparently this was not enough for him to retain his position as coach, although Boehly said that they were aimed at long-term cooperation, but apparently it wasn’t about Pochettino.