One of the main reasons is standardisation.
Where presentation matters, communication over the net is mainly done using 'standard' fonts. This is because you never know what a user has installed on their machine, and therefore you can't assume that the pretty font you're typing in is going to be the one seen by the other user. There is a small selection of fonts that you can pretty much guarantee a user will have installed (ie, they came with their computer), and those are the ones that are mainly used. Hence, you will see Arial, Verdana and Times New Roman a lot.
If you type in a non-standard font that the reader doesn't have, 'font substitution' occurs where your text is presented in a closely related font. However, this can lead to a number of unpredictable results.