I think it has to do with the fact that many epi objectives have a circumferential illumination path and many of those have larger thread diameters, in order to accommodate the illumination pathway. If one thinks of that as the norm, the epi objectives that have various fixed tube lengths and utilize axial illumination, might not seem like epi objectives. Many of them are not marked as such and also many of them that did use axial illumination struggled with internal reflection problems. Moving the illumination path outside the objective, is a definite improvement with regards to contrast.
In the 1920's, Alexander Silverman, Dean of the School of Chemistry at Pittsburgh U., invented a tiny exterior vertical illuminator( about 2 1/2" across), which was fitted around a standard objective, similar to the way ring lights are used on a stereo microscope. Spencer produced these for a few years....obviously a response to the flare encountered when using a conventional vertical illuminator, through the objective.
Am I getting this correctly, georgetmacro?