It's not taking off because, out of the box, it can't replace the gatekeepers: there needs to be a certain time period, sometimes decades, from publication until the moment a major work is recognized and cited. Cites form a DAG (old papers never cite newer ones) so do not converge on a recursive application of PageRank, newest works have all identical weights by definition.

So in the short run, you still need a prospective measure of scientific value, and acceptance for publication in a selective journal ("impact factor") serves that purpose well.