The open source community behind Selenium has a very large & engaged developer base. This ensures a stable support channel for the tool. In addition, various companies also provide support for Selenium. This active and multi-channel support provides a much more lucrative option for developers looking to implement Selenium in their workflow.

Supports many browsers and a solid platform for adding future browsers to selenium (WebDriver)

Selenium is based on Java and can be run on Windows, Mac, Linux, Unix, etc.

Testing is analogous to a human being using the web application which is not possible with a solution which runs headless and claims to run faster than Selenium.

Selenium, with its ability to interact with different testing frameworks, like NUnit or xUnit, makes it easy to get Selenium working with practically any Continuous Integration services.

The WebDriver protocol is standardized by the World Wide Web Consortium which makes it easy for third-parties to get involved and contribute to Selenium.
WebDriver has been the foundation of many additional automation projects besides core selenium and has become the de-facto standard for UI automation.

Selenium supports a variety of languages including Java, Python, PHP, C#, Ruby and JavaScript ensuring that the tool is easily accessible to a wide variety of developers.

Selenium tries to inject into the page only the bare minimum of code needed to run the tests.