You've probably been forced to leave a website because of annoying pop-up ads. Those slow, clunky, obstructing pop-ups, stop you from reading the content you specifically visited the website to read.
This isn't something that's been happening as of recent either. Pop-ups have been around for over 5 years now, with most being used ineffectively.
Most of the time pop-ups barrage the reader with deals, offerings, and calls to action. Doing this immediately, rather than letting the reader see what the site is about, ruins your chance at conversion.
You have to let your reader's read your site. It's what they came to do. After they've read an article you can then prompt them to join your newsletter.
The idea is to give them the validation that the website is worth something to them before requesting their information. In a similar fashion to how a SaaS landing page would work. You sell them what your site is all about and then ask them if they want to join it.
Readercatch (My own tool), Sumo, and Convertkit all offer what is called an "Inline email form". This is a form that is either injected into the blog post (for e.g: Readercatch), or it is copy and pasted in the article by the author (for e.g: Sumo / Convertkit).
Inline email forms are better because they don't block the content as a pop-up would. The reader can simply read your content, get a feel for what the site is about, see your call to action within the article, and choose to sign up.