It's not the curse of APD, it's the fact that Dublin is a much bigger airport with a much larger market than Belfast. Honestly I'm surprised that it took KLM and Icelandair this long before commencing flights to Europe's 15th largest airport.
If anything BHD should be congratulated for being the first airport on the island of Ireland to have KLM, Icelandair and even Brussels Airlines (albeit with less success) commencing operations.