He doesn't get the chance to

Daemon II had been trying to decide who to Fuck first when Received a Message from The Red Priest Thoros of Myr. It turned out that Joffery had ordered his Brother Steffon to execute the Lords who refused to bend the knee, and Steffon refused, this caused Joffery to mark his Brother for Death, and possibly give him an ally in the heavily contested stormlands, the problem was this: Steffon was quite possibly the only Royal Child of Cersei, that was actually Robert Baratheon's Son. Now Robert could only Legitimately hold the iron theone because the First Baratheon, Orys, was the Bastard half-Brother of Aegon Targaryen the First. And since The Targaryen's had ruled for more than Two hundred years, they were essentially the Exemplar for Royal Blood. And Robert's Father, the Namesake of his Son, steffon Baratheon was born after the First Born son of King Aegon the Unlikely slighted the Reigning lord of House Baratheon in his time by breaking a Marriage Pact, so a new pact was formed and that caused Roberts Birth and as he was a Descendant of House Targaryen through his Father, and his Grandmother was A Targaryen that legitimised his Claim to the Throne.

Daemon II had a stronger Claim to the Throne than any of Robert's Children or Brother's, as he was Aerys Targaryen the Mad's last True born Son, but their was the problem of Rhaegar's Bastard. From what Daemon II knew A select few knew the Truth, Lord Stark had to have known, and Lord Reed may have known, but Robert would have butchered the Child as a baby had he known. However as Aerys Last Son he was technically ahead of Rhaegar's Bastard, as the boy wasn't a Legitimate Son as far as he knew, and he had studied every record on the Targaryen Dynasty in the Maester's Citadel, and he hadn't read any records of a legitimate son besides the young Aegon. So with this in mind he decided to Aid the Young Wolf Robb Stark by taking Casterly Rock.