They shot throughout Sydney as well and it didnt look like Sydney. First of all the main point is money. It's cheaper to film there. Hate to break it to you but this type of decision isn't always based on artistic merit but economic reasons. Second...it would be because it's the norm to use different composites to create a original looking city. Gotham doesn't look like Chicago unless it's recognizable areas of the city but the city is much larger and more expansive than what exists in reality. Considering even most television shows use environment replacement these days it's a safe bet that there will be a composite skyline. If you think they chose Vancouver because of the great skyline, you're mistaken...it's tax breaks and logistics that made this choice. Vancouver looks like every city USA. It's generic and can stand in for most major US cities. Now Snyder may decide that Vancouver doesn't need to be touched in post but I find that highly unlikely.
