Due to the card's advantageous overuse in the popular decks featured in metagames Four-Color Adventures and Four-Color Ramp, the results of tournaments were too skewed to keep it in play in Standard format
The 6 MV is steep, but its keywords more than make up for that, especially haste
Suddenly, you have one lord and a bunch of middling creatureswhich isn't going to be as effective as you might have hoped
Then, with some careful planning, a creature will be forced to block Vastra and be destroyed, creating a Clue and a Food
I still think you might consider [casthaven]Slobad, Goblin Tinkerer[/casthaven] or [casthaven]Daretti, Scrap Savant[/casthaven] when building this type of deck, but [casthaven]Pia Nalaar[/casthaven] has a place in those decks, and can make a legitimate argument to helm it as well