Why do the Earth 2 Trade Paperbacks skip an issue?

[Mighty Roman]

I've recently been collecting the Earth 2 Trade Paperbacks, and I've noticed something strange. Vol 3: Battle Cry collects issues 13-16, and 15.1 (Desaad), and Vol 4: The Dark Age collects issues 17-20. But none of these volumes include 15.2: Solomon Grundy. Is this issue printed in another series' TPB? Or has it been skipped entirely? If so, then why has it been skipped?

[victorxd1999]

It's collected in the New 52 Villains month omnibus. It's not collected in the trades because it doesn't add anything to the story, it was only filler.