Time and money. Maybe they invested a lot in getting an app off the ground but it just didn't have the impact they expected - so they back off updates, or stop entirely. Or maybe because they're working on other issues instead that are more pressing. Or if they're an individual doing apps in their spare time, maybe life priorities changed. I'll love to spend more time on my DeekFit apps, but when I wrote them 5-6 years ago, I was single, childless, and rented. Now I am married, have a kid, and own a house. I don't have 20 hours of spare time a week to spend on my side project....so I don't.
What's sad is that we've evolved into this world where someone spends 99 cents on something and then because of that, they expect free support & updates for life. It was a dollar! When you buy a toy at the dollar store do you complain to the manufacturer if they don't give you free accessories for it? Back in the day, you spent $30-$50 on a piece of software and were lucky if you ever got a single update. The things people expect from their software today are ridiculous.