Rails 4.1 - Why Spring?

Now with Spring built-in with rails 4.1, I am curious about why the dev-team chose Spring over the others (Zeus and Spork).

Why did they choose Spring?

If you read the README from the v0.9 tag of Spring it should answer your question: https://github.com/rails/spring/tree/v0.0.9

The highlights appear to be:

• implemented in pure Ruby
• makes use of Rails' built-in code reloader
• totally automatic (boots up in the background the first time you run a command)