It's because they're in constant competition with Playstation Plus.
One month, Playstation Plus had Ducktales Remastered, Duke Nukem, Prototype 2, Infamous First Light, a good indie game known as Swapper, and a mediocre one known as Whoa Dave.
But we're talking PS3 had 3 very well known and played games and a excellent Indie title, and then the same Indie Title and a much appreciated prequel-game for the PS4.
February's included Thief and Yakuza 4, two exceptional games as well, and Transistor being an excellent futuristic RPG by the people that did Bastion.
March's had an exceptional Sherlock Holmes game, Papo and Yo (another fun one), Oddworld New and Tasty, and two entertaining Indies known as Valient Hearts and Counterspy.
And April's is best easily forgotten.
May's hasn't officially been revealed yet, but I'm curious as to what will be on it.
My point, however, is that the two are always competing. PS has balanced its Indie and AAA games and shown that it can release good ones of both for its PS+ members. The only way for Xbox to counter that is have a large list of AAA titles.
My only gripe is that PS4 sees much more indie titles than anything else, but that's most likely because it has a smaller library to grab from. I'm hoping for a complete AAA title month for PS4, but that'll probably be down the road later on.