Sigil was started by Strahinja Markovi in 2009. It started as a school project as part of his computer science course work and was the basis of his Bachelors and Masters thesis. While it was a school project there was plenty of time to devote to working on Sigil full time. Once Strahinja graduated the magnitude of keeping a project like Sigil wasnt as simple.
Simply put he was done with Sigil for a few reasons: 1) he got a job which took away most of his free time. 2) After two years of working on the same code day in and day out he got tired of it. His exact words were Ive been looking at this codebase for over two years now, and frankly Ive grown weary of it.. 3) He felt it was time to move on and contribute to other projects and use his free time in a more free manner such as working on other open source projects.