It started with bad PR and price being higher for a weaker console on MS part.
Then their 1st party support paled compared to the previous gen, while Sony (despite having a slower output in the first few years) continued to deliver what they started on the 2nd half of the PS3's life cycle.
Sony also took most of the 3rd party timed deals, which MS had the generation prior.
Then the Cherry on top was that people generally go wgere their friends are for online gaming. The gap had grown so wide that the odds that the PS4 being that platform grew exponentially higher overall.








