The Postgres crew hides behind the "replication means different things to different people, so it would be quite presumptuous of us to build it!" mantra. It's quite annoying.
You can hack together crude replication in Postgres with Write-Ahead Log (WAL) shipping. The config has some hooks in it to automate this process and bind to it. But that doesn't allow you to do circular and/or master-master replication; the receiving node has to continuously be in recovery mode.
I think slony is less popular than mysql replication because it is a royal pain the ass to setup and maintain. In mysql you flip a switch and have replication. It has a few known issues but is "mostly" reliable and understood.
In postgres/slony you enter the wonderful world of triggers and several layers of magic.