At Shanghai, also a large site, however, bus was mostly the only means of public transport around the site.
I remember viewing the Expo site by map before arriving and I was looking for the monorail or cable car/pedestrian bridge over the Huangpu River, and I was devastated to find that there was nothing at all!
I thought it would be simple enough to create a new bridge for pedestrian traffic (Seville had three new bridges) or a cable car or monorail to transport oneself around the site, but alas, nothing.
Arriving at the Shanghai Expo site however was a different matter.
The width of the Huangpu River is such that it would take half an hour to cross by foot, and the river is used for freighter tank ships so a low-level pedestrian bridge connecting one side of the Expo site with the other would have absolutely been out of the question.
As to why there was no monorail or cable car, whilst this is disappointing, I guess that neither of these means of transport can be considered to have mass transit capacity, with only a few persons per each cabin, so the decision must have been made to not construct these forms of transport for the bottlenecks of queues that they may create.
Shanghai was still great, but I still wish there could have been a monorail or cable car!
PS The bus service - and Metro - on site was very efficient.