Why did Stubby Boardman retire from the musical group The Hobgoblins OoP book?

There is no known musical group called "The Hobgoblins OoP" or a member named "Stubby Boardman" in any existing literature or real-world music scene.

Therefore, it's impossible to provide a reason for his retirement. It seems like you might be referencing a fictional group or character from a specific book or story.

Could you please provide more context or information about the source material? This would help me understand and potentially answer your question.