system
Given the following post, respond as if you're commenting on Reddit:
human
Why aren't RISC architectures like SPARC, Alpha and PowerPC used anymore?

A while ago, large companies selling non-PC workstations and servers switched to RISC CPUs, with some developing their own. Sun moved from 68k to their own SPARC. DEC moved from VAX to their own Alpha. SGI moved from 68k to MIPS. Also, IBM made PowerPC and Apple moved from 68k to PowerPC.

Back then people said CISC was obsolete and RISC was the future.  Right now SPARC, Alpha and I guess PowerPC are dead. MIPS is only used for embedded systems. Meanwhile, x86 remains extremely popular, though now running 64 bit code. 

Why did all those RISC architectures fail?

Seems like besides x86, ARM is the only other really popular and architecture used on many different devices. It's used in countless embedded and Android devices, there are ARM systems you could use as a personal computer, and now Apple is switching to ARM.

Are there technical reasons why things turned out this way?
gpt
I wouldn't say they failed you just don't see them as much in mainstream desktop / laptop machines. ARM / tensilica / AVR and their ilk's complete dominance of pretty much any embedded device goes to show that in many ways RISC won war.

On the consumer front i'll bring up one 1 technical reason and 1 non technical reason. On the non-technical front "Win-tel" (Windows + Intel) dominated the early PC space That combination outcompeted others. It became unprofitable to try and market something customer's that wasn't Win-tel

On the technical front. Intel made that lumbering dog that is the x86 ISA really shockingly performant. Under the hood these days most x86 chips are actually a good deal more RISC like then they'd admit as each of the larger x86 opcodes is broken down into RISC like "micro-ops" that are what the CPU actually executes.