GIVE ANSWERS - English

Because ”Canadian” is an adjective in this usage. The emphasis here is on the nationality.

It's also possible to use "Canadian" as noun - the emphasis being on the individual who has Canadian nationality.

So, "She is Canadian" - emphasis is on her nationality. Perhaps you are differentiating her from people in a group who have Britsh, French or Australian nationality.

"She is a Canadian" - more focus on her as an individual who has Canadian ancestry. Possibily you are describing your new girlfriend or the wife of your uncle.