It's easy to explain : many people who ask "Open World" PVP cannot be satisfied with an PVP Server or an Opt-In system since most of the time, there are not enough encounter in their opinion, plus they can't exert their frustration with gank and GK.
If they only wanted PVP, they could explain what kind of change they wish on the PVP maps but no. They ask "Open World PVP". But in "Open World PVP", there are pretty less chance to meet someone who actually want PVP while wandering on maps than when tagging for PVP maps. What will they meet on "Open Maps" : people in leveling, people doing MSQ, people gathering. They... actually expect a fair and interesting encounter? Statistics make this pretty unlikely.
Now technically, nothing in the game is built for Open World PVP : the delay for job switch and the craft/gathering gearing system are not compatible.