The main difference is whether they think that you'll write maintainable code and extensible code. Will teammates be able to work with your code easily? Will others be able to maintain or extend the code when you're gone? Is it easy to debug? Does your code use the right abstractions when appropriate? Is your code efficient? The response probably indicates that they saw something in your code that they felt wasn't up to par.

In general, this is something that you'll improve at naturally when you work on teams or maintain legacy code (assuming a relatively good codebase). One thing that can be super-helpful is to review a good codebase and see how features are implemented and think about why it was done this way and how you would have done it (for python, take a look at the flask or django codebase and see how they use abstractions). And honestly keep interviewing.