Start-up 1 (2000): Too early and inadequate access to technically sophisticated investors (located in the Midwest rather than Silicon Valley). Proof of concept in semantic knowledge graph but would have required computational resources to do it at scale that very few were building at the time. And, cloud wasn't a reality at that point. VCs started funding "semantic" about 7 or 8 years later.

Start-up 2 (2010) I was employee 7, not founder. It got through series A funding. We built it. We won award for innovation from Sloan School of Business and although CIOs of big companies professed to love the platform when they passed it off to their procurement and vendor management people adoption faltered. And, without a significant flow of RFPs being posted there was no revenue as we were going to get paid with commissions from the vendor side of the 2 sided market.

Start-up 3 (2012) - Solo founder builder. Lack of market fit. No funding, needed to earn a living so it went on the back shelf. Some technology still used for consulting clients but not as a product.

Maybe not a startup, but I created a few android apps years ago. They were not very popular. They didn't have a flashy enough interface and marketing could have been better. I assume this sort of stuff extends to startups, specifically that you need to break through the noise.

A very long time ago, we were doing automated network scans. In the early 2000's nobody seemed to care.