People already consume too much stuff a day without producing anything. That's why they don't want to subscribe to yet another newsletter. I guess the ultimate baseline of getting subscribers is getting more page visitors with SEO and continuous content production.
I have generated a couple of thousand subscribers with a giveaway ebook. [0] In the beginning, it was difficult for me to get subscribers with only content. That's why I decided to write an ebook to grow my audience, to get exposure and to learn myself more about the topic in detail.
Personally I try to keep the newsletter annoyance low for my visitors (e.g. avoiding popups to signup for my newsletter). My personal opinion is that if you are in for the long run with your website, you want visitors to enjoy the reading experience and in return to come back to your website.