They update the SDK quite regularly: https://code.google.com/p/googleappengine/wiki/SdkReleaseNot...
They rarely handle bugs well. The linked issue is typical. They don't even need to do any work to do a whole lot better; they just need to give developers an indication about what to expect. That does mean that they need to make decisions internally, which might be the problem. Potentially, management is umm'ing & arr'ing about whether to kill App Engine.
They recently threw a bit of a contribution in the direction of AppScale, but it's difficult to read that. It could be to soften the blow in the event of discontinuation, or it could be to remove the perception of App Engine as a service with no alternative.
There have also been new features recently introduced, albeit projects that seem like they should have lower prioritization than ensuring folk can use SSL with App Engine, or use modern versions of their programming language. For example, this recent addition: http://googlecloudplatform.blogspot.com.au/2015/02/using-goo...
My reading of the situation is that Google Cloud folk have resources (headcount) to do routine maintenance on App Engine, but not embark on any infrastructural advancements, like new versions of Python & Java, or significant new functionality.