This is a question I addressed during a talk at the Kiwi Book Feast event on Saturday, and here is my reply.
I write suspense because I love puzzles.
I write historical because I am a research geek, and my version of catnip is chasing the clues to obscure facts through articles, contemporary accounts, and scholarly research papers.
And I write romance because I want to show characters grappling with who they are, and nothing brings out the essence of a person faster than a developing intimate relationship.