That may account for its initial popularity but it doesn't account for its continued popularity when so many other synthesizers are available.  It is popular because it does what the most people want it to do the best.  It is responsive, pronounces more words than other synthesizers I've tried correctly out of the box, allows for rapid listening while keeping speech more intelligible than other synthesizers I've tried and it is light on computer resources.

I used DeccTalk before there was an Eloquence and, though it has a more pleasant voice, it is not as easy to listen to speech rapidly and miss as few words as with Eloquence.

Some people may say that other less human and more mechanical sounding synthesizers allow faster listening.  I can't say.  But if that is true, it is also evidently true, that one reason Eloquence is so popular is that it allows a good compromise between the sound of the voice and rapid speech intelligibility.  I suspect a lot of users don't want to use a mechanical sounding synthesizer to get faster reading speed.

Gene

I also find it interesting that the eloquence voice many people like that is also now on iPhone is now 27 or so years old already. and I think the only reason it became so popular is that Jaws started implementing it and encouraging its use back in early to mid 1996. Jaws 3.2 was the first Jaws for windows version to have eloquence. I got in on cd in the mail in July of 1996 and I thought it was amazing that you just needed a computer with a sound card and no external speech box plugged into the computer. So then eloquence took over, decTalk and Keynote gold software went away because I think eloquence was free with jaws while decTalk software and Keynote were paid products. And eloquence I guess sounded better than both of those.