Even when a relatively established technology is the best solution for a client in meeting requirements, it may be declined because the client doesn't have access to skills in that technology.
For example, I was forced to recommend Lotus Domino as the Web server for an e-business application because the client's team had plenty of experience with Domino but none with either of the two architectures I would have preferred to recommendNT/IIS using Active Server Pages and ADO or UNIX/Apache using CGI and C/Java using ODBC or JDBC. The client was adamant that the existing Web architecture should be used with ODBC access to back-end databases so that in-house skills could be leveraged, rather than retraining the staff in the use of ASP or CGI or bringing in outside consultants or contractors.
In some cases, it's important to remember who is paying for consultancy (the customer is always right). However, it's also essential to the client that the correct support for developers be available, and in this case the developers got stuck in a couple of areas of Domino development and had a great deal of trouble overcoming the problem. The Lotus team in the UK didn't know what had caused the problem, and the U.S. team's six-hour time difference made them difficult to contact. Searches of the Web came up with very few Web sites/newsgroups at which detailed support was available for Domino developers, whereas there were many for ASP developers!