For many years, toilets came in pretty much one height. When the American Disabilities Act came into effect, toilets receiving the ADA certification had to be at higher than the old standard. There are also other requirements that are well spelled out in another very recent post that you can read. As time progressed, more and more people found toilets that were ADA height were far more comfortable especially for older knees and taller individuals. For some reason or another, the "Comfort" and "Universal " height toilets came out with slight difference heights. These are not ADA because they do not have all of the other requirements that ADA certification has. So to directly answer your query, yes, there is a federal regulation for ADA, but not for Comfort or Universal Heights. Probably the tall toilets are more in demand that the older "Standard" height, but I don't see them going away completely.