The tweet listed the voice actors but it didn't state the reason why the station decided not to air Kamen Rider Gaim. The likely reason, based on what happened to Samurai Sentai Shinkenger years ago, is due to Kamen Rider Gaim's heavy Japanese elements. Take note, some South Koreans still hates Japan due to some events that happened years ago.
Kamen Rider Drive is set to premiere in South Korea in January 2016.
[Special thanks to Patrick Corpuz for the news tip!]