https://www.ancient-origins.net/news-history-archaeology-ancient-places-europe/appian-way-0021165
Surprised it took until now...
Some of it may be because there's a lot of work entering a site. I saw what it took with the Lake District.
Also there has to be a plan to help preserve it and that is tricky for big open air sites. This paragraph explained it for me. "The bid for the Appian Way to be added to the UNESCO heritage list was championed by Italy's culture ministry and supported by a broad coalition of 73 municipalities, 15 parks, 12 cities and provinces, four regions (Lazio, Campania, Basilicata, and Puglia), 25 Italian and foreign universities, and the Pontifical Commission for Sacred Archaeology,"
A miracle to get any sort of agreement from that lot 8)
this is true...