I would say that Chelsea's improvement is possible because of Enzo Maresca. Enzo is a great coach. Honestly, none of us thought that we would see such a great performance from Chelsea this season. I didn't even think that Chelsea would be able to come in the top four of the table. Due to Enzo's excellent match strategy, Chelsea's overall performance is now very stable. This team is performing well consistently. That is why Chelsea is now in the second place of the table.
Chelsea already got good foundation by their previous coach, Pochettino, but he failed to find an agreement with the club owner for development plan. They separated with a contract termination and Enzo Maresca was hired in massive doubt. Joining a big club like Chelsea with doubt but Maresca proved that he is a good choice for them at least so far and with many young players in the club, who also shown their growth in the last two seasons, I see bright future for Maresca, these players and Chelsea in general.
However, we still don't know how Chelsea will perform in the long run. Chelsea's squad is not as strong as Arsenal and City. Even then, this team is playing well. Especially Chelsea's attacking players are in good shape at the moment. Chelsea players have been able to score 35 goals in 15 matches. Chelsea is now the highest scorer in the Premier League.
Their developmnet needs time but they are growing very fast in this season, top four is in their hands and now they can focus on higher target like a second position. They are at this position already, but it's time to maintain it, defend it against pressure from Arsenal and Manchester City, as well as wait for bad results from Liverpool for opportunities of joining the title race.