Cost
Web-based and Cloud computing are taking over in the government. Trying to get money to develop anything else is near impossible. If you get past that hurdle then you have to fight the security folks for permission to put it on the network. For desktop apps the answer is "No. Resubmit in 90 days for further disapproval."
The justification for this approach is cost. Their hardware refresh cycle is now 5+ years and they don't have to buy high horsepower machines when they do purchase. The average user doesn't have any desktop apps outside of MS Office. If they were to go to a web-based office app then thin clients would be all they would need. In regards to bandwidth, they just don't care because that cost is invisible for the most part.
