The company posted its reasons and condolences on the Dynasty Warrior forums: "We know and understand this is both sad and frustrating. The closure of a game is always hardest for us at Aeria as we recognize the community's dedication towards the game but regrettably games can't last forever."
Aeria will be giving out 100% rebates to any AP purchased by players from now until the game's sunset. The company also promised that it's working on special events for the title's final days.
[Thanks to Mehighlow for the tip!]