I think Oda focused a lot more on the characters that people were interested the most in, which I assume are Luffy and Zoro. Sanji is probably ranked a number or two above him, but a small gap like that can make a big difference.
Oda has already given Luffy and Zoro a lot of development and special attention, so development for characters like Sanji and Nami are slower, but noticeable when they happen. Sanji’s newly awakened abilities during Wano and Nami‘s improved Clima-Tact with Zeus are good examples.
There‘s also characters that Oda has had on the back burner for a while, such as Chopper and Usopp, with the latter being on the burner the longest.