Because it is free, NVDA has a broad and diverse user base and is also the most commonly used screen reader among accessibility testers and developers, since anyone can install it without a license cost. It pairs well with the free Firefox and Chrome browsers for testing and receives frequent updates from its maintaining organization, NV Access.
Screen reader usage surveys, such as WebAIM’s, have shown NVDA and JAWS trading places as the most-used desktop screen reader in recent years, which is why credible accessibility testing generally covers both rather than relying on just one.