The annexation was part of a general turn in French imperial policies between 1805-1810 which incorporated various satellite republics and kingdoms directly into the French empire. The 1805 transformation of the Ligurian Republic into départements of France set the precedent for incorporating non-ethnically French territory directly into the imperial metropole. Illyria fit within this trend, but was also a special case in that it came after the War of the Fifth Coalition in 1809. The defeated Austrian's ceding of this territory meant that they would no longer have a direct access to the sea and hence no direct connection to the British.

Sources

Connelly, Owen. Napoleon's Satellite Kingdoms. New York: Free Press, 1966.

Dwyer, Philip G. Napoleon and Europe. Harlow, England: Longman, 2001.