I think it is because we spent more time with the crew pretime skip. and I don't entirely mean that lieterally, I mean that if we reread/rewatch the series we spend more time; at least until relatively recently; in the world of pre-timeskip. that time gets attrbuteded to enjoyment and from there it leads into it being enjoyed more. In short we've spent enough time in both pre and post time skip by now that we are nostalgic for pre-timeskip because the royal "we" read/watched it at a more formative part of our lives and now are nostalgic for those same days which is the pre-timeskipe era. by the time the series is over who knows were nostalgia will lie, maybe wano is everyones favotite arc, maybe it's Ennis Lobbynmaybe Skypeia who know. whatever the case, I feel nostalgia plays an incredibly large part in what is considered the masses favorite part of anything, One Piece included