From my personal experience, people are more likely to read and share threads for a number of reasons:
* They don't have to go off platform.
* the base tweet can be much more eye catching that a link, even with social cards set up
* People may be desensitised to links that lead off-platform, because that's how Ads generally appear on Twitter
Then there are other advantages, like the fact that it tends to encourage you to write in a concise and direct way, with each tweet in the thread being like a bullet point. I often see people retweeting and engaging with a specific tweet from the thread; it's harder (relative term) to do that with a section from a blog post.
I think it's primarily because you put actual content right in front of them, increasing the chances of them getting hooked by quite a bit. Before your blog post has any chance of hooking people, they have to first take the plunge of clicking on it. Threads eliminate that friction.