Hi Andreas,
Thanks for your question. This is due the font licensing. Typekit doesn't own all of the fonts in our font library; they are owned by our foundry partners, who work with Typekit to make their fonts available to all of our subscribers. Some foundries have chosen to offer all the weights and styles of a family to sync, while others include a selection of weights and styles.
We are continuing to license fonts from our foundry partners for sync as we move forward. In the meantime, if you need a particular weight of a font that isn't available to sync, you can likely purchase a license directly from the foundry. The foundry name is listed at the top of each family detail page on typekit.com.
If there are particular weights or style of a family that you'd like to have available to sync, you can post that list here or email it to us at support@typekit.com and we'll see what we can do!
I hope that this helps; let me know if you have any other questions. Best,
-- liz