Probably got wipped out or didnt manage to get though the elite four to challenge her. Even in the games legendaries aren't that strong outside the average trainer battle except maybe ones around the end game. Also she took out Paul's entire team with just her garchomp without breaking a sweat. So it's best to assume her levels are very high or maxed out.
I just assume everybody including Ash and and Paul equivalent to Victory Road trainers and Tobias was the fag that got through it with his legends before realizing the elite four takes more than catching legends to be strong filler in your team.
I could also just suck at vidya when I was a kid by doing the same.