All is not functional, nothing is technical only: there is always a direct or indirect impact on users, positively or negatively. The poor quality of a product impacts on the time-to-market and the reliability of features, and therefore on the end user satisfaction negatively.
No hermetic barrier exists between the bottom and the top. When too much technical debt is accumulated, it becomes bugs directly visible to users. Some changes made in code in debt may be even enough to make a first bug appear. Moreover the degree of achievement while implementing complex user stories may depend on making proper technical topics before.
User stories and bug fixes deal with short-term goals, while technical topics and technical debt reductions deal with long-term goals. It is the reason why these last ones are so important.