We can blame the French. If we hadn't stolen the French word "environs" (meaning "surroundings") to base the word upon, we'd have not had to cope with that hideous extra "n".
The Greek word for "surroundings" is "Geo", and I think perhaps we could have done better if we'd used it. Perhaps by sticking an "n" on the end huh? Geon? Geoment? Geonment? (Just for you).
Too late now.