The comma makes it look like you are using direct address (or "vocative case"). Quoting from the site that I have linked:
When addressing a person or thing directly, the name used must be offset with a comma (or commas if it's mid-sentence).
So if Mary has done something and I want to know why, I could say,
Why, Mary? Why did you do that?
Note that the comma separates the name of the person who is being asked the question.
Therefore, if you write "Why, Subaru?" then it looks like you are addressing the company named Subaru and asking them why they did something. In the tweet you link, the writer says:
Why, SUBARU, couldn’t you pay a native English-speaking copywriter (or any native English speaker) to check your content before launch?
Here he is using direct address to ask Subaru a question.
If you want the meaning "Why should someone choose Subaru?" then you should not use a comma and just write "Why Subaru?" so that you are not putting it the form of direct address.