This issue is caused by a limitation in the Gallio adapter NCrunch is using to discover Xunit tests. Basically, the adapter doesn't search nested Xunit classes for tests when using Static Analysis. A workaround is to set the 'Framework utilisation type for Gallio/Xunit' solution-level configuration option to 'DynamicAnalysis'.
I'm currently looking for ways to properly solve this problem. It only affects Xunit (not the other frameworks supported by NCrunch).
For anyone interested, a fix for this problem has been introduced with the newly released version of NCrunch (v1.45), so it's now possible for NCrunch to discover nested Xunit tests using static analysis.