Actually, there are (have been) several commercial implementations on .NET:
- Bitkoo (acquired by Quest then Dell, now defunct)
- ViewDS (Australian-based, integrated with their virtual directory)
- Axiomatics (both Java and .NET) (disclaimer - I work there)
In the open-source space, there are definitely more Java engines than .NET engines but that is likely due to the nature of the languages. Java is more prevalent than C#. There is even a PHP project for XACML.
(Source)
That said, the implementation language shouldn't really matter to you. XACML offers authorization as a service that sits within your infrastructure and can be easily consumed from multiple languages. XACML defines a REST/JSON interface which you can easily use.