Learn German: Für wen zeigt der Fan diese Bewunderung? - For whom does the fan show this admiration?

This is because of the noun die Bewunderung (admiration). Just like in English, this noun requires a specific preposition to connect to its target. In German, the fixed phrase is die Bewunderung für, and the preposition für always triggers the accusative case. Since we are asking about a person, we use für plus the accusative question word wen.

Why can't we use the compound question word Wofür here?

You would use Wofür if you were asking about an inanimate object, an idea, or an action (meaning 'What does the fan show admiration for?'). Because the admirer is directing this feeling toward a person ('For whom?'), you must split it into the preposition für and the person-specific pronoun wen.

Can I put the preposition at the end of the sentence, similar to the English 'Who does the fan show this admiration for?'?

No, German strictly forbids 'preposition stranding.' When a preposition is tied to a question word like wen, the two words must stay glued together at the very beginning of the interrogative clause. You can never leave a preposition dangling at the end of a German sentence.

Why does the English loanword Fan use the masculine article der rather than the neuter das?

While many English loanwords in German do default to the neuter gender, loanwords that refer to people usually adopt the masculine gender as the standard, unless they specifically refer to a female person.