While marriage doesn't make teen pregnancy better, there are two aspects of that I'd like to address. First, like others have mentioned, in some cultures it is normal to get married and start a family at a young age. In these cultures, the family unit is seen as extremely important. If this is the case, barring abuse/neglect, the child is usually in a better position being raised by both their parents in the same home (put down the torches, I say this as someone who comes from a broken home and has NOTHING but respect for the single mother who raised me).
There is also the fact that I feel teens who get married probably take the responsibility a bit more seriously than others. Not all, of course. But a teenager who is willing to take another big step and get married to provide their child with a good home most likely will do everything they can to provide the best for their child.