Corrections in red.  Stylistics in blue.

Why aren't some of my notes/posts corrected?

Is it because they are very long?
Or maybe, there aren't enough native English-speaking people to correct all the people who are studying English? )

I will not speak about your posts, but about posts in general.  

I think you have correctly identified two reasons why not all posts are corrected.  It can take as long as 20 minutes to carefully correct one lengthy post.  I will occasionally do that, but on some occasions, I simply don't have the time.  So I correct one or two shorts posts instead. 

You may wish to break down a long post into two or more short posts.  I suspect that the shorter posts will have a better chance of being corrected.

And yes, most people on italki study English, and sadly, there aren't enough native speakers to correct each and every post written by non-natives.