All these new frameworks are designed to work with OSS tech. So lets say some better NoSQL platform comes out, it supports OSS technologies first. .Net comes later and sometimes it is difficult to decide which client library to pick to connect to the NoSQL service. OSS client libraries would mature early and .Net libraries would take a little time.

Connecting OSS tech and MS stack is getting easier but still leaves you with a lot of uncertainty.

This is an example: http://wiki.basho.com/Client-Libraries.html These guys have Erlang client but there is no support for .Net.