They left him to die out of cruelty. Robin was presumed dead, years long gone to the Crusades, nobody would have expected him to return and leave a message for him. A blind old man left alone in a burned out castle, far from civilization, would not survive long. It was blind luck that Robin returned in time to find him.
4th wall break inside a 4th wall break that's like 16 walls!

The Sheriff and his men feared Duncan's mullet.

The trivia section is user submitted, it's about as accurate as Wikipedia.
There's a moral to this story Del Boy but for the life of me I can't find it!

Ok I found the reason why. From the trivia section.
Apparently Duncan had his eyes removed because he didn't want to believe the accusations about Lord Locksley. Probably some BS lies that the Sheriff made up because Locksley didn't want to join them. And the fact that he's the only servant left on the Locksley estate when Robin arrives suggests he might have been the only one who resisted. So most likely the other servants joined the Sheriff or they would have suffered the same faith.

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No, another servant betrayed Locksley. Understandably to save his own skin.
Duncan tried to stop the other man from bothering Locksley.

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How would they know Robin would ever return, or in time?

Mabey they wanted to send a message. Duncan was already old and killing him would be too easy. So they left him to suffer and for Robin to find him.