I would say that the biggest factor why IT projects fail would be lack of buy-in from management. This really influences most other obstacles, which could in itself be seen as reasons for project failure, but are really only symptoms of the above. These could be anything from insufficient resources, be it people/infrastructure/time to bad project management. The project manager could make or break a project and should anticipate risks, deadline misses, scope creep and all project related issues. Another person who could make or break the project is the Project Sponsor. The project sponsor should be picked very carefully and this person should manage communication from the project to the Board of Directors / Management / etc. and communicate all important and urgent issues raised by the project manager to the directors and manage their expectations. In my experience these different people influence the success of a project much more significantly than any technology issues.