Because I had some gender concerns, didn't know how to feel. But then the idea of being non-binary came to mind, and it clicked immediately. Whereas now the idea of being considered to be a cis man feels wrong. So calling myself non-binary feels emotionally correct and is accurate to who I am, while my past identity (well the cis part anyway) would feel wrong and be wrong. It is the correct label for me. There's other specific and/or related identities that also arguably or definitively fit me, but the broad term of non-binary is also the aspect of my gender identity that both comes to mind for me the most and is how I'd bring it up and call myself in most places.