After the release of Angular 2, the team decided to go with semantic versioning. Semantic versioning follows three number versioning, representing major, minor, and patch. Patch version is the last number in the version that will be incremented for every patch release, usually bug fixes. Minor version is the middle number in the version that deals with the release of new features or enhancements. Finally, the major version is the first number in the version that is incremented for the release with breaking changes.

Angular team switched to use TypeScript 2.2 from TypeScript 1.8 that is used in Angular 2. This introduces some breaking change that obviously leads to increment the major version number. Also, the current version of router module is 3.3.0, which is not in alignment with the other modules of Angular that are still in 2.3.0. So, in order to keep all the module versions in sync and follow semantic versioning, the Angular team decided to go with Angular instead of Angular 3 for their next major release.