Make money with blog + adsense?

Hey guys, I opened a new blog like 1 month ago so I told to myself: lets monetize it. I sent my request to adsense and after 1 week they answered me:
Your AdSense upgrade request has been denied
reason: insufficient content.
Now my question is: why ? I have 45 posts published. Some of them have 300 words. Some of them have 500+ words and some of them have almost 1000 words.