Good question...I'd speculate that as the various manufacturers submitted their pistols for the JSSAP/XM9 trials in the late 1970s/early 1980s, the white dot sight became more common among service pistols. Trijicon's website states they introduced "the first tritium-illuminated iron sights for handguns" in 1985, and they were adopted by the FBI for issue on the SIG in 1988. I imagine the FBI adoption further opened the door for widespread issue by US law enforcement.