This bid has a lot to do with European idealism. For much of the 20th century, these three countries were divided by politics, yet they are now EU partners, and they--especially (north-)western Slovenia, Friuli-Venezia Giulia (Italy), and Carinthia (Austria)--form an interconnected region between the Alps and the Adriatic. It's not unusual for people in the area to have lunch in one country, go shopping in another, and then go back home to a third.
It's also the only place where the three major European language groups--Germanic, Slavic, and Romance--converge, as do the continent's major physical features--the Alps, the Mediterranean, and the Pannonian Plain. In several respects, the area is Europe in miniature, and this idealistic (perhaps too idealistic) bid would underscore that.