Originally Posted by
Konachibi
Because usually when an MMO is released it's in it's finished form, and the player-base consists of people looking for content that the game holds, and then from that point on things are simple added that usually revolves around that kind of content.
In the case of FFXIV, because it's being 're-made' from it's original design, it allows Yoshi to see what parts people do and don't like about what the game currently holds, get rid of things that nobody likes, tweak things that people do like, and then add things that people want. People on this want both open-world and instanced content.
In the case of Warcraft people wanted mostly just instanced content, so world bosses and things were eventually phased out as no one bothered with them. Warhammer Online concentrated on open world PvP aspects because that's what many of it's players wanted more of, and so on.