Mercury lamps just happen to emit light at 253.7 nm, very close to the wavelength of UVC that best kills microbes, 254nm. 185nm is also regarded as a wavelength

Mercury lamps just happen to emit light at 253.7 nm, very close to the wavelength of UVC that best kills microbes, 254nm. 185nm is also regarded as a wavelength that kills microbes, but this is by proxy of it being the wavelength best at producing ozone, which then kills microbes. It has been possible to purchase LEDs with a peak wavelength of 255nm (250-260nm) and other wavelengths of UVC and UVB for a decade at least, but they cost about $300/pc iirc, making them very expensive for building a one-off microbe-killer flashlight or disinfecting multi-LED array. Comparatively, Mercury lamps are cheap.