Workstations are now almost-extinct form of computers. Basically they used to be high-end computers looking like desktops, but with some important differences, such as RISC processors, SCSI drives instead of IDE and running UNIX or (later) NT line of Windows operating systems. Mac Pro can be seen as a present form of workstation.
Mainframes are big (though they do not necessarily occupy whole floor) computers. They provide very high availibility (most parts of a mainframe, including processors and memory, can be replaced without system going down) and backwards compatibility (many modern mainframes can run unmodified software written for '70 mainframes).
The biggest advantage of x86 architecture is compatibility with x86 architecture. CISC is usually considered obsolete, that's why most modern architectures are RISC based. Even new Intel & AMD processors are RISC under the hood.
In the past, gap between home computers and "professional" hardware was much bigger than today, so "microcomputer" hardware was inadequate for servers. When most of RISC "server" architectures (SPARC, PowerPC, MIPS, Alpha) were created, most microcomputer chips were still 16-bit. First 64 bit PC chip (AMD Opteron) shipped over 10 years after MIPS R4000. The same was with operating systems: PC operating systems (DOS and non-NT Windows) simply were inadequate for servers.
In embedded systems, x86 chips are simply not enough power efficient. ARM processors provide comparable processing power using much less energy.
