Jazz, the dialects of Scots vary so much because of the different linguistic influences of the regions. In the north, they had previously been Pictish speakers, then Gaelic before they spoke Scots, with a heavy Old Norse influence (i8E314 is right in their observation above). This made their Doric dialect very different from the Lallans dialect in the south, e.g. Edinburgh, where the previous language was Cumbric, Gaelic was at least a second language for many, so that when Old English came along it was rapidly heavily flavoured with words from these languages, as well as Dutch, Flemish, and French due to our trading alliances with Northern Europe. This is where Scots diverged from Old English and went on to evolve in its own, unique way.
Some maintain that Doric should be considered a separate language from other dialects, but I'm not convinced the differences are sufficient to justify it.
There are also distinct Shetlandic and Orkney dialects who previously spoke Norn, as well as a south-west dialect where they previously spoke Cumbric and Galwegian Gaelic, a very distinct Gaelic dialect that was almost another language in itself.
The last branch of Scots is Ulster Scots, sometimes called 'Ullans', which came about with the influx of Scots speakers to Northern Ireland during the plantations, and again evolved in its own way.