> When they were the only 64 bit systems

They were never the only 64 bit systems. MIPS introduced their 64 bit R4000 in 1991, a year before the Alpha came out. Sun released the 64 bit UltraSPARC in '95, along with IBM's 64bit PowerPC AS for their AS/400 systems. HP released the 64bit version of their PA-RISC in 1996.