Because GW2 wasn't designed like GW1. Instanced content is probably easier to make varying levels of difficulty and those who wish to engage with higher degrees were able to do so. The open world concept of GW2 probably wouldn't allow for this segregation. Increasing difficulty affects everyone, as was shown with the initial release of HoT. And we all know how well that went. Players, such as myself, aren't making excuses to not have harder content. Instead, we don't see how Anet can provide such with limited resources that would take away from the existing model of the game. True, they made raids, but those were released with an expansion. If players are going to get any additional "harder" content, then it will probably be in instances like raids.