Back in the Windows 3.1 days, websites didn't have all the flash, images, etc that they do now, so dialup was plenty for what was avaiable then. Also, file downloads were usually less than 1 MB...so they didn't really have a need for anything faster than 28.8K.
There is still a fair amount of people running Windows 3.1 in my area though and its a pain having to try to fix their computers. I learned the hard way to take a copy of 3.1 when going to repair somebody's PC because one time I took only a copy of Win98, and behold, my customer's computer was using Windows 3.1.