uh, Windows does support other architectures than x86: currently, it supports x86, x64, ARM and ARM64. (and as Windows Server 2008 R2 is still supported, the IA64 version is also still suppored)
ReactOS isn't driven by the same market forces. Anybody with enough determination can port it to SPARC and fly away :)
Windows NT 4 supported x86, MIPS, PowerPC, and Alpha. All but one were dropped with Windows 2000... and it wasn't until XP that non-x86 returned (initially IA-64, and x86_64 in three more years after initial release).