I believe it's because he simply doesn't want to.
In the series, he flashes his eyes to people who have committed some sort of sin, such as the man on the street or the little girl in the school who had bullied Chloe's daughter.
He appears to do this out of a mixture of anger and amusement. Whilst we don't know the final effects on the bully or the man on the street, we know from the Pilot that exposing himself to Jimmy Barnes led him to near insanity. Now, he obviously showed his whole face, not just his eyes, so this exposal was much more frightening, but it's still worth noting the effects.
With Linda, he doesn't really need to show who he truly is. He is having sex with her in return for some psychological help and has no need of her beyond that. He could expose himself, but why bother? He could lose his psychologist and it doesn't in anyway benefit him.
With Decker, he's currently quite confused about her character. Remember, she appears immune to his charismatic "charm" and doesn't reveal her desires to him the way others do. He's fascinated by this. Again, he could show his true self to her, but he risks scarring her, losing her (which he definitely doesn't want), and not understanding exactly why she's so immune to his abilities.
Given the potential negatives of exposing himself to Linda and Decker, particularly since he appears to care about both of them to a certain extent, it seems these negatives far outweigh the potential benefits of exposing himself.