Since they control it, they view the card as weaker, which means it stays out for longer
The way it works, is when Thassas Oracle enters the battlefield, you get to look at the top X cards of your library, where X is equal to your devotion to blue (how many blue mana symbols appear in the casting cost of all permanents you control)
Sword of Once and Future shows up in a lot of EDH decks, but not coherently aside from Mirko, Obsessive Theorist
After some testing of this card, I can report that it is simply worse than Mana Short
This two-drop 0/1 flying Bird Pirate cannot block, and then when you deal combat damage to a foe with a Pirate, you explore here once for all of the damage dealt, and then that's just once per combat step unless you have one with double strike or at least two dorks one with first strike and one without