Well...a DECtalk also weighs about 15 lbs and is packed with components. So it was probably a weight/quality tradeoff considering it needed to be mounted on a wheelchair.
I understand Hawkings last version use a Raspberry PI, with the original voice.
I was a kid when I saw one, in Brazil, at a computer convention. It was, probably "Feira Internacional de Informática" in Brazil, in the early 1980s. The DEC team was incredibly friendly, even to two kids (I went there with a cousin) who would, maybe, be their customers, 20 years later.
Dr. Dennis Klatt. He developed cancer (throat of some sort, I'd assume) and did lose his voice but died within a year, AFAIK.
Perhaps more memorable than the very notable DECTalk, he built original voice synthesizer for, and was the voice of, Stephen Hawkings.