Hey,
The main reason why we need to rename or move that file to another directory because, the default zookeeper installation (tar extract) comes with the conf file named conf/zoo_sample.cfg while the same extract's bin/zkServer.sh expects the conf file to be called zoo.cfg thereby resulting in a "No such file or dir" and the "failed to write pid" error.
So before running zkServer.sh to start or stop zookeeper instance, either: rename the zoo_sample.cfg in the conf dir to zoo.cfg.