First, because AdSense is a very simple and proven solution that already pays for hundreds of thousands of blogs/websites accross the world!
Also, AdSense is provided to you by Google which not only makes it reliable but also trustworthy. Once you have earned money from Google, you can actually trust them to pay it to you!
Finally, because the ads will be contexual, i-e they will be relevant to the content of your blog/site. This way, your visitors will not be offended for seeing the ads on your site.