This symphony only has two completed movements, and just why this is the case has long been a vexing question for musicologists. There are sketches for a good portion of what would have been the Scherzo movement, but nothing at all is known to exist for a fourth movement -- maybe. It's all very strange, really -- this symphony isn't like Mozart's Requiem, left incomplete due to the composer's death. Schubert did die young, but he still lived another six years after he completed the two existing movements of this work. Some believe that another piece, in the same key and using the same instrumentation, was intended as the symphony's finale but was then used instead as a piece of incidental music for a play, but there just isn't enough evidence in favor of such a hypothesis to really make a strong case. And then there are some -- Leonard Bernstein was one -- who believe that Schubert simply decided that the first two movements were sufficiently good that he didn't need to write another two. I find this idea a bit hard to believe. Schubert was a pretty staunch classicist and tended to stick to established forms, for the most part, and in any event, he did complete another four-movement symphony after this one, his own Ninth (which is often called simply "the Great").
My own guess is that it's a combination of the above. Other projects came along, maybe he had ideas that he ended up using other places, maybe he always intended to get back and finish this one but then he got sick, and so on. We'll almost certainly never know, unless some kind of evidence -- a letter in Schubert's hand, or maybe even scores to two never-known movements -- turn up in some old attic in Vienna.
I don't really know a great deal about Schubert, to be honest. I've never explored his music much beyond the last few symphonies. He's best known for his art songs, which is a part of classical music that doesn't always engage me all that much (my failing, not that of the art song). Nevertheless, this work is an intriguing bit of music history. Schubert wrote it in 1822, but it wasn't even performed until the 1860s!