Labs that offer E-4 are much more likely to be serving a market for processing old film. It's fairly common, in countries that have had family photography for generations, to find film in an old camera, or unprocessed rolls in a box or drawer -- and if the film is older than the current process, you may be dependent on a lab offering an obsolete process if you want to attempt (in this case) a color slide as opposed to a black and white negative. In the United States, there is (or was, ten or fifteen years ago) a small sub-hobby of looking specifically for old cameras in thrift shops, yard sales, and such -- in search of film that might have been left in them for decades. I have personally processed black and white films that were fifty years old -- but black and white process hasn't really changed since before 1900, where there have been at least four color negative, and at least three color slide processes since the Second World War, and the new films don't work in the old process (and fairly often, the old films don't work in the new -- like C-22 Kodacolor in C-41).
The big difference between E-4 and C-22, as obsolete processes go, is that E-4 films were much less "birthdays and holidays" films, and much more "travel and professional" films, thus more likely to contain images where the color is important enough to spend the cost of an obsolete process.