I read in the 5 March 2005 issue of "Amateur Photographer" (UK) magazine that the reason this is happening is because Fuji can no longer get one of the key raw-materials needed to make the film. This is the statement from the Professional product manager. See page 4 if you have this issue.
I assume by the content of your post that Fuji Velvia 50 is to be discontinued. I also assume by yours and other posts regarding Velvia 100f that this magic ingredient is absent in that film and therefore is velvia in name only.
Fuji did announce a month or so ago that they would discontinue Velvia 50 by the end of the year and replace it with the Velvia 100 - which is a different film from the 100F. The Velvia 100 has been available in Japan for a couple of years and isn't new, except to the US/UK market. Fuji says they will continue marketing two separate 100 speed Velvia's. Here is a link with an article about the new film: http://www.kenrockwell.com/fuji/velvia100.htm
It was discussed previously in another thread, but this is the first time I have seen Fuji announce the reason they are discontining the Velvia 50; I felt that Fuji's statment warranted bringing up the subject again.
Nice to hear that Velvia ,no matter if it's rated ISO 50 or 100, won't disapear from the market. I've tried to use Kodak ExtraColour 100 instead of Velvia ( it's much cheaper) but it was not even a little bit like Velvia...