When you boil down the digital assistant race it really boils down to developers and ecosystem. Google Home is looking to leverage its Android army. Apple hopes to do the same by reportedly opening up Siri to third parties. Microsoft has the Windows 10 and Cortana link.
Alexa's milestone is notable since Amazon has enlisted big name companies to build apps and highlights its early lead. Capital One, Domino's, Fitbit, PGA, Samsung and Uber are some of the companies building Alexa apps.
In addition, Amazon has tied Alexa into its Amazon Web Services. Specifically, Alexa is tied to AWS Lambda, which runs developer code and compute resources are automatically managed.