The software crisis of the late 20th century led to questioning of the value of the Waterfall methodology. Waterfall is the most well-known example of a class of methodologies known as sequential methodologies. Following the publication of the Agile Manifesto in the early 2000s, many commercial organisations began using Agile methodologies. There are many Agile methodologies which form a class of methodologies which are called iterative methodologies. Iterative methodologies have had some, though not complete, success in commercial settings. There is significantly less evidence of public sector organisations adopting iterative methodologies. Since the introduction of iterative methodologies, a further class of methodology has emerged which incorporates elements of both iterative and sequential methodologies. This class is called hybrid methodologies. There continue to be examples of projects in the public sector that have failed, overrun in scope, budget, and cost. This work explores the reasons behind the reticence of the public sector organisations to adopt iterative or hybrid methodologies and analyses the factors that can affect IT project managers’ methodological choices. Eight project managers working in either a Swedish municipality or a United States (U.S.) federal agency were interviewed, and their responses analysed with a thematic analysis. The findings are that all the surveyed project managers were using hybrid methodologies and that the factors affecting methodological choice are external factors, such as law, compliance requirements; the political nature and organisational culture of public sector organisations; the ambiguous role of the project manager in public sector organisations; and the project’s nature. Other findings were the emergence of cyber security as a constraint and the lack of maturity of public sector organisations in relation to project management.