Following is some of the cases where competing software won this or that battle, and I am jealous since I want everyone to use what I like:
- Git (and GitHub) is so much more successful than Mercurial (and Bitbucket), my preferred VCS. I mean even the website looks better.
- Markdown is a far more popular light-weight markup language than the far more advanced reStructuredText. It's particularly sad when it's not used where it's needed most... documenting large projects (example).