Also to the list of reasons why, last gen the 360 had a year headstart in the US, cheaper retail price and easier to develop for hardware. This helped MS build momentum very fast with zero competition and secure a lot of 3rd party support. This gen they have non of those advantages.
Secondly, Shareplay - playing games you do not own for free with friends.
Thirdly, ongoing exclusive content support - whilst Sony almost released an exclusive game every month in the last 2 years of the ps3, ms almost dropped all support bar a few games. That didn't help MS along with the other issues.
Last but not least, Sony has released more Exclusives including F2P, Indie and AAA than Ms has so far and in the first Qtr of next year, they are already looking at 2 big AAA releases. You also forgot they have recently released DC, Tlou, infamous first light, Samurai Warriors 4, Singstar, and soon will have Lbp3.