PS4 is like the Wii all over again. People aren't buying it because they are interested in the games. They want it because it is a trophy and say "I have the must have piece of technology." Also like the Wii, Sony cannot keep up with the demand and it is constantly sold out.
Xbox One on the other hand is the PS3 of this gen. Most people don't want the thing because it is overpriced and always on shelves. Because of those 2 things the perception is that it is not a good way to spend their money.
For both of the consoles it is a cycle where PS4 will continue to gain popular opinion and the XB1 continues to fall behind. However just like 7th gen, (assuming both consoles survive to the third year) the numbers will even out and then both will sit on 100 million units sold.