Because GoodsportsMANship is sexist?
Good Sportsmanship Trophy not available
So that's who's been hoarding the Good Sportsmanship trophies - I wondered where they all went!
Seriously though, that's strange - did you give it a minute to refresh and check again?
Maybe that is one of the ones that is only available from the game page. Try going to a game you played with him/her and click the link to give a trophy from there.
That even works better if it is the game you wanted to provide the trophy for.
I went to a players homepage to give him a Goodsportsmanship Trophy and it was not listed as one that I can give. Why? I see he already has 18 of them, why can't I give him another one?