They never let it in the kernel. I personally believe that some ext people were even scared of ReiserFS's efficiency and didn't want it in the kernel. Not monocausal but this was also a reason.
Now, for being developed by so few (one?) persons, and being out of tree it still makes a good run. It's sad that it never really had the chance to go mainline. Ext was never _that_ great and btrfs is still fiddling with some issues and still not announced as ready for productive systems (but I'm glad Chris Mason is not at Oracle anymore).
But so there are some things, people, good ideas, sometimes ahead of their time. Masses laught at them, but much later everybody uses the same principle and agrees that this is now common standard.
Stop TCPA, stupid software patents and corrupt politicians!