Usually it's one or more of several common factors: cost to make, cost to buy, cost to integrate, slower/inefficient performance, loss of compatibility, some unintended design defect, poor or over-ambitious marketing, or some assumption that compatible/optimized/"unobtanium"/Carnot-efficient software will solve Issue X for us. See Itanium, DEC Alpha, Transmeta, Consumer Power/PowerPC, OS/2, Commodore, PCjr, ETX motherboard standard, Java Processors, etc.
