The Akroma/Spirit tactic was chosen for the Meandeck version of Oath because let's say that one turn four or so you play Oath with Forbidden Orchard (pretty typical).
Turn 5: Oath up Spirit of the Night. Attack. At some point use Forbidden Orchard again.
Turn 6: Oath up Akroma, Angel of Wrath. Attack with both. You've dealt 18 and most decks use fetchlands or CoB or FoW to do 2 to themselves. If not, you'll still win next turn unless both of your creatures are killed.
Whereas the Colossus version goes...
Turn 5: Oath up Colossus.
Turn 6: Attack for 10 (they have a free blocker from the Orchard).
Turn 7: Attack for 11.
The Colossus version would do more damage later on, but it wouldn't matter, because the opponent would already be dead.
Plus if you get a Lotus you might actually be able to cast Akroma or Spirit, while Colossus is still going to be another two mana to pull off...