Thery have built experimental reactors and run the tests. Why aren't we leading the way with the first full scale?
We (both the DOE and private companies like Westinghouse) are putting money into R&D on it but it's not ready for prime time yet. South Africa is scheduled to begin construction of the 1st "utility" scale demonstration PBR this year with operation in 2010. Another factor is optimum size PBR technology. Every thing indicates that somewhere between 100 and 200 MW is the max compared to 1000 MW+ for light water reactors so basically you need to build 10 PBRs to get the same energy that one PWR or BWR can deliver.
Until we get some experience with this thing, I would not think it wise to gush about it. When things scale up in size, there are always suprises. If you recall, back in the 60s and 70s, HTGR's were all the rage until they actually built one and ran into a host of problems they never anticipated. Light Water technology may not be as sexy as the new stuff, but for now, we know PWRs and BWRs work and work very well with no suprises.