Decoration. Some time after the Emperor was enthroned, his custodes came to believe that the poor fellow looked a bit dour, so they decided that a bit of sprucing up might turn that eternal frown upside down. Didn't work, though; as it turned out he's just a cranky S.O.B.. There's just no pleasing some people.
Maybe they wanted to install a second one in the first place, but then they couldn't afford it for budgetary needs (money is a real issue in an universe where a guy just sits on a throne of gold, believe it or not), so they just went for the "old One Eye look"
Maybe the Emperor had suffered an injury (or some other reason to add the bionic eye) previously and the eye was of such fine craftsmanship it wasn't obviously a bionic until, well, the rest of him rotted away?
Well he was not a rotten corpse going onto the golden throne, Horus ruined one of his eyes as well a lot of the rest of him, I'm sure his minions did their best to patch him up before placing him on the throne, also some artwork does not show him with a bionic eye. Also the emperor can still communicate with people albeit rarely and may have wanted a bionic eye to survey the real world or people who have access to him while the majority of his body decayed
Little-known bit of lore: an intern Custodes, during his first day on the job, tripped and accidentally jabbed his spear into the Emperor's dusty old eye socket. Quick-thinking, he covered it up the gouge wound with a monocle and claimed, famously, "He was like that when I got here. I don't know what you're all going on about."
I am not impressed by the heretical, blasphemous ridicule being directed at our beloved Emperor.
I have all your names and addresses, so you will all be receiving a visit real soon by some extremely 'ard blokes in golden armour and f#ck-off halbards!
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