1. You did not join the event prior to its conclusions. Your player may be the Top Scorer in your League but if you have not joined the challenge you will not get the reward and as soon as the season finishes you can not join the event retroactively any more.
2. You forgot to claim the reward. The rewards are not auto-claimed.
3. Your player is not the Top Scorer: Top Scorer is not chosen after your last match but after the last match in your League. It might happen that you were close to winning the challenge and was in first place after your last League match but some other player beat you last second.