I want to know the answer to the following question:
What was the main cause of the spread of Shtokavian?
All the background I'm provided with is that "Due to population migrations, Shtokavian became the most widespread in the western Balkans, intruding westwards into the area previously occupied by Chakavian and Kajkavian (which further blend into Slovenian in the northwest).". Is that enough to answer the question?
Answer: yes