1. No actual customer. You built something you THOUGHT people wanted, but they actually didn't.
2. Bad/No Marketing.
You need to talk to customers to discover a problem first, then work on fixing it. In all my time trying to do side projects I'm becoming convinced this is some sort of innate skill as I have acquaintances who seem to have an uncanny ability to spot an opportunity in just about anything. If you can pair with people like this, great!
As for marketing, this is even more key and possibly even fixes problem 1 in some cases. I've said this before, but take a look at many of these "I started a dog social media website and now I have 10k MRR" more closely. 90% of the time the author already had a blog about dogs or something similar with 50k subscribers.
To that end, I've been focusing on working on my blog :)