Windows shops are vary of writing functionality, that might one day compete with Microsoft. Writing net Tomcat/JBoss could clash with IIS and future frameworks. Something like this happened with nant/msbuild and nunit/mstest. Commercial companies find it difficult to justify development of tools with high risk of being obsolete soon (something competing with MS product does not stand a chance in most Windows shops). Apache projects do not care about this risk.
It is surprising that some of the apache projects have .NET equivalents (lucene, log4j, ant) but some others dont........