For small-cap companies, debt instruments can be a perfect way to raise capital. Investors hold a promissory note promising to re-pay the capital loaned, with interest, over a specified period of time.
If a company can get debt financing, it's a great choice for them. But there's not a lot of investors interested in loaning money to a company with no assets and little to no revenue. At least not at reasonable interest rates. And unreasonable interest rates can be problematic due to usury laws. The rigid repayment timing can also be problematic for the borrower. Equity is really a better fit for companies that have greater than, say, a 10% chance of not being able to repay their investors.