Since the vast majority of colorless spells are artifacts, this severely limits your design options
Three mana puts a Shock on each of your opponents, as well as an edict effect thats guaranteed to hit a threat
Thats because, in many cases, its going to be the last card played in a game
That meant they would force the opponent to have to block, so why not give the Boros a mechanic to help win those fights
Ricardo Silva White creature decks have historically mostly either been interested in taxing your opponents resources with cards like Thalia, Guardian of Thraben and Leonin Arbiter, or in playing cheap creatures in an aggressive style of play (White Weenie)
And for this reason, Ghostly Flicker has more possible applications when compared to the competition