Luke’s astral projection should have been his green lightsaber form. He lost his father’s lightsaber on cloud city.
That bothered me a lot as well. Kylo Ren himself witnessed his grandfather’s lightsaber being torn in half moments earlier, so seeing his uncle wielding it should have tipped him off, and I also think the green lightsaber would have unsettled him a lot more since that’s the last image he has of his uncle.
I think it all boils down to Anakin’s lightsaber being more iconic.