I think it's mainly due to the market having settled on two instruction sets: x86 and ARM. Ten years ago creating ISA-specific code was a much more risky decision because it would inhibit or complicate porting to one of the other architectures (PowerPC, SPARC, Itanium, Alpha and MIPS were still viable -- now only PowerPC remains as a distant third in the ISA contest, holding niches in game consoles and IBM's high-end servers).