Here's why:
* "Greek" is the most commonly used and accepted term. It's direct, respectful, and avoids any potential for misunderstanding.
* "Grecian" has a somewhat archaic feel and is often used in more formal or literary contexts. It can also sound a bit pretentious.
* "Greek" is a self-designation. People from Greece identify as Greeks, and using that term acknowledges their own preferred identity.
Therefore, it's always best to refer to people from Greece as Greeks.