Because if the game they're making isn't guaranteed to be a mega hit, Sony won't market your game well, if at all.
How the hell do you get to that conclusion?
All-Stars got some ok marketing push. Infamous started out slow and Sony kept marketing muscle behind it to build the series. Uncharted had a strong first entry but not a "mega hit", Sony's marketing was a big part of the second and third games selling so much better. Twisted Metal had a lot of marketing for it, the game just didn't click with consumers. Same with Starhawk, which came on the heels of Warhawk's lackluster sales.
In a generation where games like Brutal Legend are marketed as something other than what they really are, strong titles like Darksiders 1 and 2 go with basically zero marketing behind them, etc., I'd say it's pretty hard to depict Sony as a publisher who doesn't do a better than average job at trying to market new titles.
Their marketing approach isn't always great, that I could understand a complaint with, but it's not due to a lack of effort. Hell, All-Stars had a holiday season clear of other major first party titles and still couldn't move units. It's just a genre/type of game that doesn't fit Sony's core audience.