Why is Bulgaria considered a springboard beyond the EU?

Because the EU single market and non-EU markets border each other directly here: the Turkish border is a good hour's drive from Burgas, Istanbul's major airports four to five hours, Serbia and North Macedonia via the A1 Trakia motorway, the Danube links to Central Europe, and a weekly ferry runs from Burgas to Georgia. The full details with driving times are on the Invest page.