I took a look at the newsletter subscription statistics for Biology.SE and to my suprise, not a single user was subscribed to it. To make sure that this is a bug and to exclude the possibility that maybe just nobody want to read the newsletter, I tried to subscribe to it myself.
I went to http://stackexchange.com/newsletters and looked for the Biology site under the Science category, but there is no Biology site listed there. I checked all of the other categories as well and even used the search, but the Biology site is nowhere to be found on the newsletter page.
The absence of the Biology site there goes a long way to explaining why nobody subscribes to the newsletter.
Counting the sites on the newsletter page, I get 75, while on http://stackexchange.com/sites there are 84 sites listed. It seems that Biology isn't the only site missing the newsletter. Taking a closer look, it seems that none of the sites younger than Movies & TV appear on the newsletter site. All of those newer sites are missing.
The option to sign up for the newsletter in my own profile also doesn't appear on Biology for me.