"That trick never works!" - Rocket J. Squirrel
or
Single-purpose machines built by lone geniuses for other lone geniuses don't scale.
The Xerox Star was an attempt to commercialize the pioneering computers that emerged from Xerox's PARC labs in the 1970s. They started with machines called the Alto, and continued with Dorados, Dolphins, Dandelions, and Daybreaks, which came to be called D-Machines, all running (mostly) the same software. They were a marvel for their day, and had all of the components of today's windowing PCs from Apple and Microsoft, but years earlier. The story of how the Star came to be and how Xerox came to abandon it is a complex one, and like the system itself, the full story is distributed across many obscure places that are not entirely consistent with each other.
But I think that the big picture reasons for its failure get lost in the soap opera details, and are worth a few words. Obviously, Star had to be released too soon for its technology. Many of the reasons for its lack of broad adoption can be traced to the speed of its underlying electronics; and much of the architectural and usability weirdness of the various models of computer in its history can be traced to attempts to compensate for this basic problem. Fifteen years later, Apple introduced the Macintosh, with almost the same capabilities, but with a commodity microprocessor CPU instead of custom designs made from bit-slice processor elements, at a tenth the price. The Star had died out by then, but the Mac continues on to this day.
What kicked the Mac over the top into the realm of success was its developer ecosystem. Technically, the Macintosh and Star developer models were very similar. In the early days of the Mac, it wasn't possible to use it for development of its own Mac software -- developers had to use the substantially more expensive and powerful Lisa, and then test and distribute their applications on a separate Macintosh unit. This was the same process that had been used for the Star, where developers used a Mesa development environment, and then deployed the results to the Star environment.
The difference was that there were already a lot of Apple II developers who were excited to move up to the new and awesome Lisa system. I was a grad student when the Lisa was released, and one of my friends got one -- he must have spent a substantial part of his meager life savings on it. The business and research communities that were available to Xerox in its own time could not match that kind of commitment.
The community that developed the Star was small and exclusive. I happened to be attached to a PARC-adjacent computing community in the mid 1970s and there were rumors of amazing things going on over there, but exactly what was never clear. It was impossible to tell from the outside, but much later it emerged that the PARC computing community was internally even more fragmented than it appeared to be This turned out to be typical for an entire stream of computer research history, but the phenomenon was particularly acute at PARC, and its effects showed up in fundamental aspects of the D-machine architecture.
Notably, there wasn't really a "D-machine architecture". There were high-level visions of what might be possible in a world where every person had their own computer beside their desk, but everyone had a different version of that vision. PARC had been staffed with some of the most brilliant, creative, and independent computer researchers to be found, and they created Promethean hardware frameworks that could be elaborated into whatever their application vision required.
The result was that a given D-machine could become one of four different computers, depending on who was using it at any given time. A relatively tame instance provided for programming in the BCPL, Mesa, and Cedar languages under the control of a command line interface in its windowed development environment, Tajo. Starting the machine with a different microcode load would turn it into an office workstation providing a suite of word processing and publishing tools with the now-familiar windows, icons and folders metaphors, but without software development ability. Other microcode created a Lisp Machine running Interlisp-D, or a dedicated Smalltalk machine. The microcode and even the device drivers for each environment were unique and incompatible and the user communities for each one didn't seem to talk to each other, and their writings rarely even acknowledge their counterparts' existence.
A D-machine gave its user a vast amount of control and flexibility -- it could become whatever you could imagine, if only you had the talent, skills, and time to build it before your tenure at the lab expired and you had to go somewhere else. Collecting brilliant people and giving them access to the most advanced tools and letting them work on whatever they think they can be the most productive at is a standard strategy for top tier research labs, and it often produces marvelous results. PARC produced many marvelous things. However, creativity is unpredictable, and unpredictability is largely unworkable as an element of business strategy.
Xerox management turned out to be unable to solve the problem of herding its houseful of cats in a coherent direction that led to business success, and their inability to settle on a common machine language that would be tolerated by all their diverse projects is emblematic of their failure to achieve focus. Each external developer community was too small to provide Xerox with the volunteer support that it needed in order to attain a critical mass that contributed to business innovation in the way that DECUS or SHARE did for DEC and IBM, or that Apple enjoyed in a less organized way, instead of draining energy and R&D funds. Whether that critical mass was possible given the other limitations of the technology is impossible to say, but there's no evidence that Xerox or PARC management had any notion that customers could be creative contributors to a product in a way beyond passive consumption. It would take another two or three technology cycles before the idea of a "platform" became a common sense aspect of business strategy.