According to the game's director Hidetaka Miyazaki, Bloodborne couldn't have come out on previous consoles. "I always wanted to make a game set in the Victorian era, but to make it look good—with all the clothing and the architecture and things—required a console more powerful than the ones that were around. the arrival of the PS4 finally gave us that," Miyazaki said in an interview with PlayStation LifeStyle when asked why the game was a PS4-exclusive. "Bloodborne isn’t a game that could have been cross-generation, it was only possible on PS4."
Of course, that doesn't really explain why it's not coming out on Xbox One or PC, but that probably has more to do with Sony's negotiating skills than technological limitations.
Bloodborne will be released on February 6 in North America and Europe, a day after it's release in Japan.