There's always more to be said on any topic, but the main reason for debate is modern Turkey's denial of the incident. There's a whole section of the FAQ where you can learn more about it https://www.reddit.com/r/AskHistorians/wiki/faq/europe#wiki_the_armenian_genocide

Two threads you might be interested are this amazing one with discussions by /u/Georgy_K_Zhukov and /u/yodatsracist covering general questions about the genocide https://www.reddit.com/r/AskHistorians/comments/1pxtql/questions_on_the_armenian_genocide/, and in the second one /u/yodatsracist discusses in depth on why the topic can be controversial.