Time.
Time is crucial to building design expertise because two of the most critical aspects of building it are time-constrained: practice and curating a repertoire.
#1 Practice. To transform design practice into expertise, one needs to finish projects. By completing several projects, one works with designers with different approaches, interacts with stakeholders from diverse backgrounds, and receives feedback from people using the final product. Finishing projects takes time.
One also needs time to process the feedback and the outcomes from design projects. The design process can be fuzzy and non-linear. It may require some time to organize a portfolio, weaving design stories into narratives (design cases) to learn from the journey.
#2 Repertoire. One needs a broad, multidisciplinary repertoire to become a skilled designer. To create it, one needs to practice curating skills: filtering valuable content from noise and building bridges among them. To do that, balancing critical thinking with creative thinking is fundamental. While one helps link content layers, the other helps create unique perspectives and concepts and turn them into design products.
Doing such a thing consumes a lot of time. Building a repertoire and applying it into a design practice takes years.
Doing both is a lot of ongoing work. The good news is practicing design and curating a repertoire build upon each other.