Apparently it is because of his relationship with Thalia.
Source: Walker Scobell Teases a Darker Story in Percy Jackson Season 3
Original Question: How do you feel that [Walker] evolved as an actor from Season 1 and how has Percy evolved since Season 1?
You can clarify in terms of personality, in terms of relationships with other characters, backstory, other aspects you have in mind, or all together.
I know he doesn't use any of that in Rebels, but I think that could be fixed with him having some sort of character development in the show that allows him to use it and then something happens that undo's all that character development. Or maybe he uses it a few times and then he stops because it reminds himself of Palpatine. Or maybe he just develops it for the show and never uses it again, it's not that big of a deal that we need an explanation. I just really want to see Maul use lightning, I think it would be cool and maybe he could use it to help him in future fights against Vader. I honestly thought that in Episode 8 all the visions and flashbacks would lead to some revelation where he could use Force Lighting, and that's how he defeats the Inquisitors after nearly losing, but that never happens.