Maybe it's unlocked by extreme need?
Actually, maybe all the players have to find ULTIMATE POWER on their own terms? Remember the sequence where Dave got the legendary epic weapon right before falling asleep. When he asked Davesprite about it, he speculated that if John had been tugging at it with his Pure Heart and hero spirit and shit, it would have popped out like a champagne cork, but Dave had to break it. Maybe this is indicative of a larger pattern?
John is the naïve protagonist type, who gets his superpowers out of nowhere by REALLY, REALLY NEEDING them. He'll just naturally pull some wind powers out of his ass at a certain point during his quest, and it turns out that the power was Inside Him All Along, etcetera etcetera. Dave and Rose go about it a different way. Dave gets the Time Tables from his future self who grinded for four months and probably had to go through all kinds of ridiculous shit to get them. He essentially cheats by means of time loops, fitting for the Knight of Time. Rose's power as the Seer of Light would thus be knowing everything there is to know about the game, which she gets by studying Ancient Unspeakable Forbidden Lore and forming dark pacts with Horrorterrors.
Even if it really is that simple, I have no idea how Vriska would have gotten her Luck Beam. Maybe she got it from her Denizen after killing it? That would be the standard RPG procedure. Kill nemesis, take loot, gain powers. And she's a Flarper, a powergamer and no stranger to grinding, so something simple would be appropriate.