It's a Sprint exclusive in the US, and Sprint did not even sell any Windows Phone 8 devices during the last NBA season. NBA.com - NBA Mobile If you click on the Android app link, you'll see in Google Play that Sprint is required.
My guess is that there will be a US Windows Phone 8 app for next season, since Sprint now has Windows Phone 8 devices. Whether or not the app will still be a Sprint exclusive for all US smartphone platforms is too early to tell, since NBA doesn't start until fall.