Because no one knows.
Here's what is known:
1. He likes it in Finland and plays for a team where he gets to play on the top line and in every game situation, so it's very good for his development. The AHL doesn't seem like it holds anything additional for him, other than adjusting to the NA game. Past interviews suggest that he is not interested in the AHL and would like to continue to work on his game in Finland.
2. He attended the Islanders prospect camp this summer, so he definitely has aspirations to come over at some point.
I personally see him continuing to develop in Finland until he is ready to take a spot on the Isles roster. When that is, is anyone's gues - probably 1-3 years. But hopefully sooner than later!