Because the web is largely build on the http protocol and its request & response mechanism, dynamic and quick access to data has been initiated on user interaction – Click and wait for new page to load and render with new data.
Historically, the possibilities of any real typographical design on the web have been very limited and interactive designers certainly haven’t had it easy for the past years. It’s a shame really and when compared to what you can do in printed media, the web world is a mess of broken standards and workaround. Same goes both ways, as print designers have had problems learning to cope with and adjust to the new more limited world of the web. Still this is 2011 and we are all still wondering why designers, regardless of background, cannot have full control over their typography.