Tell me in Java, without looking up any online resource, how to set up a FastCGI web application. Can you? Probably not. Now tell me how to write a static method in Java. If you have at least the equivalent Java experience of a freshman undergrad, you can do the latter but not the former.
If you think Lambda should use FastCGI as its common interface, then by all means create a project on github that deploys a Go-based FastCGI bridge to Lambda and makes it easy for you to bundle up your FastCGI application into a Lambda function. Problem solved! Now you just have to get people to use it.