It's expensive, so demand is low. Demand is low, which keeps the price up.
But basically, its competitor, DDR memory, gives better value for money. RDRAM works faster overall with heavy memory loads, but DDR is more efficient with intermittent lighter loads. For the average user, DDR is more than good enough for him. Given the price difference, people opt for the DDR platform. Intel was forced into supporting the DDR platform on its P4s, and now it's effectively abandoned RDRAM. For the recent 800MHz FSB CPUs from Intel, e.g., Intel has not made any chipset which supports RDRAM.
I think Rambus is supposed to have improved the initial latencies on its more recent 32-bit 4200 RDRAM modules (don't know whether yours is one of those), but by now ddr has become the mainstream.
RDRAM's edge in performance over DDR is also beginning to disappear. The 800MHz FSB 3.0GHz cpu with dual ddr400 beats the 533MHz FSB 3.06 with RDRAM 1066 on almost all tests. That's perhaps not comparing like with like, because one has a faster FSB. But Even with the same cpu and same FSB, the 3.06GHz (533 FSB), a system using dual ddr333 also beats a system using the RDRAM PC1066, on most tests. click here scroll down to click the links to the benchmarks.