To answer your last question: 1) The type of land in which you are building. Some can't support high-rises or can only do that at great expense (which defeats

To answer your last question: 1) The type of land in which you are building. Some can't support high-rises or can only do that at great expense (which defeats the purpose of building high). 2) Infrastructure. It's easy to cram a lot of people into one building but it becomes harder to alleviate traffic or ensure access to things like shops and health care.