I think the main reason was independence from vendors - since they were based on NT and Microsoft just pulled the support - ouch!, not necessarily nicety or features of a desktop. The whole infrastructure was changed and not just some frontend.
"Pulled support" may be a bit hyperbolic, still, microsofts announcement to EOL Windows NT was a major point in the decision making. There's an interesting detailed document here http://joinup.ec.europa.eu/software/studies/declaration-inde... that talks about this and other aspects of the Limux project.
> Until 2003, the city was using Microsoft Windows NT 4 across the board, and was by and large satisfied. When Microsoft decided to end the support for this operating system, this meant that hardware and important procedures would eventually stop working. “It was from this experience of being totally at the mercy of an external party that we wanted to take the road to more independence”, Schießl (ed. the deputy project manager) says.
So having seen the effect of a vendor lock-in, the conscious decision was made to move to an environment where vendor lock-in can be avoided. It may take considerable effort to support a linux distribution after its official EOL but it certainly can be done - unlike any other closed-source OS.
