Because of patient advocacy.
Professional associations focus on career mobility and protecting their respective practices.
Many unions, while not historically, but certainly currently, narrowly focus on issues of pay and benefits for their members. CNA does both, and how we do it seems to have captured and kept national and international attention. The medical industrial complex doesn't like this. They would prefer we fall in with a more accommodating model that makes way for harmful restructuring, downsizing, workforce deskilling and the rationing of patient care. That's probably why you elected them.