Why are people saying that he didn't explain it? It's right there.
They would have to develop the Vita version separately, which would have meant a significant increase in porting costs, whereas the Wii U, PS3, and 360 versions would share assets, engines, etc. The Vita and the 3DS are also very different architectures from the Wii U, PS3, and 360, and from each other, too. So when they decided to port it to Wii U, PS3, and 360, Vita development wasn't an obvious option - it would have needed to be considered separately.
Note that I'm trying to elaborate on what he said, so it's more clear to those who didn't pick up on it directly, hence why it's not a once sentence explanation.