I think its about how many exclusive franchise a console has.. Halo was the hottest franchise leading upto 360, then gears happened. But in mean time Sony was losing previous exclusive franchise, but they were busy buying studios to deliver franchise after franchise, Uncharted, Infamous, littlebigplanet, the last of us, resistance, Motorstorm, Demon Souls (spiraled into dark souls).. these are all franchise started just on PS3.
And Sony turned on Beast mode in terms of releasing exclusive by the mid ps3 life cycle, as opposed to MS who didnt had enough first party firepower to support 360, which was dwarfed by ps3 exclusives. And MS had to drop 360 support in order to focus on xbox one.
So I think it was the momentum from 7 gen which helped Sony lead the sales with PS4







