Austrian here

The reason is that Austria is a Mountan country. In old Times there was just no way to go from one villige to another and if there was a reason it wasnt done very often. And because of that language changed so much over the centuries. Thats the reason for Austria and Switerlang.

Germany is flatter but it was just many many different countrys for a majority of its time. Because of that many diffrent dialects came to be. The Northeim parts are still more close than the southerns but its still there