Eloquence continues to be popular for some valid reasons. It is easy to understand at extremely high speech rates, its pronunciation is usually excellent and the pronunciation is also consistent and predictable. It also doesn’t tend to interpret most abbreviations, such as reading dr. as drive or Doctor. The same cannot be said for more human-sounding synths which are not always consistent in their pronunciation, even adding extra phonemes or letters to certain text strings not in the text.

I like Eloquence. It is the easiest to understand with my hearing loss.   

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