Resource allocation?
This is purely speculative, but I figure MS thought they'd be better off focusing their attention on hammering out exclusives for the next gen to entice buyers of the new console vs risk seeing declined sales on the tail end of the current generation.
If Ryse didn't get pushed to the X1, and say Forza (Not saying Forza 5 was ever a 360 game, just using as an example) was another 360 game, the X1 would be seeing a lack luster launch. Holding them back for a new console and let support fall for the old console could very well see more new consoles sold.
Sony on the other hand gambled on more last gen sales, releasing exclusives late in the game like the Last of Us. Now imagine if Last of Us was a PS4 exclusive title available at launch instead. The PS would have a more enticing launch lineup, but waning exclusive support for the PS3.
It'd be a gamble either way for either manufacturer. Each likely weighed the pros and cons, and each took a different approach.
Sony is seeing a benefit with the PS3 with continued sales late into the generation, but it can also be seen as hurting the PS4 with fewer exclusives in comparison, which could sway some buyers one way or the other.
Xbox on the other hand is hurting late gen as many resources would have been allocated to the next gen instead, but can be rewarded with a more varied exclusive lineup early in the game for the next gen.
As a note, this is all speculation, I really don't know either way, but this is the way I see it.