Being digitally competent is an imperative requirement for the 21st century university teacher, a fact recognized by both the literature and policymakers. Although this topic has been addressed in different reviews and critical studies recently, none of them have systematically and explicitly addressed the factors that explain, or are explained by, the digital competencies of university teachers. Examples of these factors include, among others, demographic, professional and psychological aspects of university teachers, as well as very specific digital competencies. The present study seeks to close this gap through a systematic mapping of the literature published until 2021 in journals indexed by Scopus and Web of Science (WOS). Based on the selection of 53 primary studies, we characterized the literature and summarized the main results reported so far. The analysis allowed us to conclude the following: (1) there is a growing number of contributions aimed at understanding the acquisition of digital competencies, especially from external factors; (2) European, and more specifically Spanish, university teachers from multiple disciplines are the most studied population; (3) most studies adopted quantitative approaches to explain but not prove causality; (4) there is a great heterogeneity of relationships and results that explain the digital competencies of university professors. The implications of these results are discussed with a view to identifying the gaps that provide scope for future research.