Because cyano is a ubiquitous hitchhiker unlike 99% of all other reef tank invaders that must ride in and get in mainly on reefing or marine-only transfers. cyano you can input from garden dirt under a fingernail you didn't clean off well enough, then it capitalizes on what overfeeding was there
cup test:
if you set a cup of water on the windowsill whether freshwater or sw, and keep it hydrated and open topped for a year, and add fertilizers to it occasionally it'll be a mix of red and green cyano depending on conditions, molds, fungi, all cycling in the cup and all you ever did was add clean water and some powdered ferts, no actual monerans. they get in, they travel worldwide...some on aerosols or suspended flocs on the air, its all quite masterful of them. that gnat that landed and drowned, he was in the garden yesterday and from that he brought many hitchhikers...cyano included among the offenders of clean water.
in that cup, we could never ever get valonia or invasive bryopsis or macroalgae or dinos, the obligate hitchhikers. At least in the Texas panhandle we could not get them by air