simply because it is not good enough to make the cut in most decks
Sorecery speed makes it far worse than the other tutors, so when you're cramming all the cards you want into your Vintage deck it's often one of the cards that doesnt make the cut.
You dont usually want to Imperial Seal for a card then pass the turn. Bad stuff can happen in the meantime like the opponent plays a Chalice that locks off the card you tutored for, or they play Jace the Mind Sculptor and fateseal the card you tutored for to the bottom of the deck. if you have Imprial Seal in your deck you likely play a bunch of cards that let you draw on your turn like Preordain, Top or Gush.
All that said, I like to play cards like Gush, Top and Preordain, so I like to have Imperial Seal in my deck =)