Commercial Macsyma is ... a special, and rather sad case. As explained to me by Danny Hillis in 1982-3 when he wished my company, LMI, could provide Lisp Machines for Thinking Machines, Inc. to help develop the Connection Machine 1, one motivation was Symbolics' pathological business practices, and the nastiest example then was Macsyma. Back then the MIT Technology Licensing office was still horrible, and it was arranged that Arthur D. Little, I think, recommend how Macsyma be licensed, and it ended up being exclusively to Symbolics, which was not a common approach.
As far as we could tell, Symbolics bought it primarily to keep it out of the hands of LMI and anyone wanting to run it on conventional hardware, and went so far as to try to get people who have Vaxsyma copies to send them back and stop using it, which was not well received as you might imagine. That helped Fateman, using the DoE which had sponsored much of the work, to force MIT to release a snapshot as open source, which eventually became Maxmia.
In house, Symbolics treated it with benign neglect, and I don't think it was massively improved. This situation became ironic as their hardware business declined and the Macsyma unit became an important cash cow, but as these things go, for the usual internal political reasons, it never got development resources commensurate with its actual status and potential.
MIT also didn't reward the people who'd originally written it at MIT, something Joel Moses, who I happened to be directly reporting to in the 1987-8 time frame when he was the EECS department head, was obviously not happy with, along with I'm sure many others. So in short, Symbolics did nearly everything they could to mess up the Macsyma community and product, and the noted decline, once there were good alternatives, was inevitable.
RMS is not 100% wrong in his loathing of Symbolics....