Because Latias and Latios are allowed. The Soul Dew easily makes them overpowered. It basically gives them +1 SpA/+1 SpD (the equivalent of a single Calm Mind) with no drawbacks. Soul Dew was banned when Latias was allowed in standard, and the Specs set was still pretty destructive. Now imagine that without being locked into a move AND having tons of special defense. And the fact that Latios has 20 more special attack than Latias.
And the reason they're allowed is because the other non-event base 600s are allowed.
although quit frankly, the battle frontier fucking deserves getting soul dew draco meteors being spammed in their faces considering how it's fucking coded