Why does 兴奋 (excitement) not mean what I think it means?
The major reason is that 兴奋 is generally used of people, and most commonly in the phrase 令人兴奋 lìng rén xīngfèn. It can also be used of other biological entities in a scientific context, closer to English excitation  (and the original derivation of the English word excited).

The original sentence has 足球的兴奋, which sounds odd because of 足球 not being a "person" of any kind. Not strictly incorrect, but definitely odd enough to merit correction.

Another possible, but more minor, reason is that 兴奋 is strongly associated with the word for "doping" (兴奋剂 xīngfènjì, literally "stimulant/excitatory chemical") in the sports context. Perceived in a very negative light of course.

乐趣 is a lot more appropriate for this: it is not necessarily as "childish" as the English word "fun", and can include the feeling of "excitement".