South Slavic historically formed a dialect continuum, i.e. each dialect has some similarities with the neighboring one, and differences grow with distance. However, migrations from the 16th to 18th centuries resulting from the spread of Ottoman Empire on the Balkans have caused large-scale population displacement that broke the dialect continuum into many geographical pockets. Migrations in the 20th century, primarily caused by urbanization and wars, also contributed to the reduction of dialectal differences.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What caused the dialect continuum to become fractured?
migrations from the 16th to 18th centuries resulting from the spread of Ottoman Empire on the Balkans