When it's an agile project, either devs will choose to do the right thing, or too many developers will work on new features, whether because it gives them more visibility or because they like working on features more. And when it's not agile, either you have a product/team manager that understands the importance of not drowning in tech debt, or you have a manager that constantly prioritizes features over quality.
Neither approach guarantees that the right choices will be made, just that in Agile, the developers mostly have themselves to blame.