User blog comment:Pon-pon/Why Oda Why?/@comment-3181121-20110111112518

I think there are several reasons:

1. Originally Oda didn't really have devil fruits in One Piece, IIRC in romance dawn luffy got his powers from the gomu tree and abilties were somewhat unique.

2. Oda wanted the reader to be relaxed even during the fights, that's why it's a somewhat wacky ability. Luffy with Akainu's df wouldn't make for a fun reading experience.

3. It is a good devil fruit, but not "godlike" like WB's or most the logias, so Ruffy actually has to do a lot of work and "fight with his heart". Besides. It kinda fits to ruffy's personality...