He dropped it because of moral shackle, Sanji can use it to peep at women.
Nah kidding, he dropped it because he doesn't want Sanji to be stronger.
Who knows but it's clear that something made him change his mind.
During WCI - fans were not expecting sanji to get RS because of his feud with Judge way of doing things.
Oda still gave him one.
Then we were not expecting Sanji to use it until dire situation.
Oda made him use it against Page one.
And, when we expected him to use always
Oda - 😑😑😑😑😑
I think he changed his mind because of fans not liking Sanji using RS...