Because when you stack lifts on top of each other in different parts of the shaft you can have multiple lifts running at once, hence multiple time the efficiency per m2. (High latency though)

Lifts are also limited by speeds, if they go too fast they make some people queasy. This is also an issue to contend with since express elevator are limited in the 'express' bit.

Edit: to be a bit clearer if you split up a shaft into 4 sections with fours lifts you can have four lifts full of people being moved, hence your bandwidth is 4 times as much. Taken to extreme you have one for every floor http://en.wikipedia.org/wiki/Paternoster

Lifts on the ground floor have to service the entire tower, where a lift that started half way up the tower would only have to serve half as much traffic as the ground floor lift, so you could have like a dozen lifts that go from ground->50%, and 6 lifts that go from 50% to top.