Pyroar was pretty great because it's immune to attacks from basic mons, which include almost every EX mon. Some attacks ignore effects on the defending pokemon, so they can pierce through Intimidating Mane, but otherwise he was pretty solid. In later sets, spirit links started happening and they kept making more and better Mega pokemon and giving them a lot more support, so Pyroar isn't as effective anymore. It can still be used well, but it's not a complete game-changer. That's my most basic understanding of the situation.