He also thought that being the world's first 100% organic country will increase tourism. He had stated in his manifesto that he wants Sri Lanka to be a 100% organic country.

So I would say his reasons are a mix of saving dollars, trying to increase tourism and religious delusions by him and his soothsayer (Gnana Akka). He had no idea it would result in a food crisis and was misled by people like Vandana Shiva that it won't cause yields to drop

I don't think he was following any "new green deal" agenda like some conservatives say but it's possible that he did it to try and gain sympathy from the west as a "climate hero" since he's known in the west for being an alleged war criminal.