From Software aren't a big studio. They had to partner with Sony to release Demon's Souls, hence why it was an exclusive. Dark Souls has a lot of rough edges. Namco Bandai will have given them a lot of money for Dark Souls 2, but it'll only go so far. As PC is the lead platform for Dark Souls 2 development, and the Xbox One and PS4 are very PC-based, it will be relatively easy to port DS2 to next gen. All we need to do is let them know before and after release that we want ports and they'll do their best, like they did with the PC version of Dark Souls. Just let them finish the game first.
Massively Multiplatform since ever, fanboys!