DUB has cruise ship traffic and scenes from Game of Thrones. Zagreb has... nothing of interest to Americans.
90% agree..... Nothing of interest to most Americans especially non-millennial. Its not a big capital city when compared to other cities in Europe and its tough for them to travel to USA due to economic struggles in joining EU.
However, ZAG can make a nice little three-day stop. You can do day trips to Plitvice Lakes and Slovenia from it too. Food was excellent (especially the Croatian favorite: the struk) and I had one of the best airbnb's for the buck.
What would become of these routes the day the GOT fad is over? That seems to drive a good deal of the US-source traffic.
Though Dubrovnik is a city of 30-40K - it serves a wider area called Dubrovnik Riviera, Montenegro, and parts of Bosnia. Day trips are available for Bay of Kotor-Montenegro, Mostar-Bosnia, and even Albania. Dubrovnik is the wealthiest part of Croatia though everyone who lives there makes their money during the high season. They do a lot of Split-Dubrovnik or Dubrovnik-Kotor mini sailing cruises now. I don't think any major cruise lines start or end cruises in Dubrovnik.
It may be tough to sustain especially for US travelers since I have heard there is a tourist tax implemented in Fall 2017 that added 30% to every food/drink bill. This makes other European beach resort areas cheaper (or more cheaper - I found Crete, Madeira, and even Santorini cheaper than the Dubrovnik Riviera).
If I ever wind up on a flight to DBV again- I am most likely going southeast to Montenegro for the week if I can't get to TGD or TIV cheaper or more conveniently.
Airlines flown: A3, AA, AC, AF, AM, BA, B6, CA, CO, CX, DL, EA, EL, IB, LH, MI, MQ, NH, NW, NZ, OU, PE, QF, S4, SQ, TP, UA, US, VS, WE, WN
2019: CX BOS-HKG, WE HKG-HKT, CA HKT-PEK-EWR, B6 EWR-BOS