You might be surprised, it's not because of an incompetent team (don't worry, you're amazing!) or running out of cash (yep, there's plenty out there). The top reason is because there is no product market fit. This means you're building a product that no-one needs, or the market doesn't understand.
So before you go spend two years in producing your masterpiece, investors have given the advice to build a quick prototype (minimum viable product) and spend some quality time doing market research. Figure out early if you're solving a problem that has a market.