I totally understand what you are saying here (for me, replace .Net with
Java).  A lot of factors influence the choice of platform, including the
ability to find people to maintain it, and the ready availability of
support.  The .Net and Java ecosystems are just bigger than the Rails
ecosystem is at this point, so going with those platforms is a
respectable decision for many organizations.

Personally, I think that the choice to use Rails has to do with
professional evolution and the desire to be able to serve your clients
better in the future, by being able to delivering solutions of high
quality in a reasonable time.

Wes