1) I suppose the high rate competence is the first and biggest reason why blogs fail. When blogging was a newborn activity, it was very easy to catch people's attention and maintain users interested, in spite of not so much effort by the webmaster, A simple blog explaining how to clean one's house caught attention, maintaining users engaged. Now that exists a very fierce competence, even niche topics got depleted
2) costs: do you remember until 2015 it was really possible to monetize a blog for free? Still no cookie laws, privacy, GDPR and whatever else European Union comes up with. To maintain a blog in accordance with the law, it costs. If we aren't programmers, it costs, as we must pay a web agency. This expense comes in addition to domain name and hosting. As in the beginning nor domain name nor hosting were really essential (blogger from Google was enough by itself), today some experts say they are fundamental (unless we are not able to run Wordpress for free) to prove the professionalism of the blog, due to the tons of blogs existing now. So blog fail when the webmaster can't afford all this any more
3) laziness making a blog to fail exists and mistakes about the kind of blog to open exist too: mistakes about the right target (audience) to reach, the niche, etc., but in my opinion nothing compares to the n. 1) and 2)