It wasn’t a commodity! It was a single-owner proprietary technology. You needed a license from Rambus to manufacture it. And at least at the time it commanded a premium price over true commodity DRAM.
People on Slashdot got really worked up when Intel signed a deal with Rambus to make RDRAM the only supported technology on Intel x86—from which they relatively quickly backtracked.
Anandtech (sniff) of course has great contemporary background:
https://www.anandtech.com/show/545
