First, because it's yet another dependency. Consensus-based systems like CockroachDB, Dgraph, Cassandra, Riak, Elasticsearch, ActorDB, Rqlite, Aerospike, YugaByte, etc. are wonderfully easy to deploy because they can run with no external dependencies. (Their consensus protocols may have different problems, but that's beside the point.)
Secondly, it's a pretty heavy dependency. The JVM is RAM-hungry and it's difficult to ensure that it always has enough RAM. Running multiple JVM apps on a single node must be done carefully to make sure each app has enough headroom. It consumes considerably more RAM than Etcd and Consul.
Thirdly, I think it's fair to say that ZK is showing its age. It's notorious for being hard to manage (see the other comments in this thread), with a fairly old design (based on the now-ancient Google Chubby paper) that, while resilient, is also less flexible than some other competing designs.
Secondly, it's a pretty heavy dependency. The JVM is RAM-hungry and it's difficult to ensure that it always has enough RAM. Running multiple JVM apps on a single node must be done carefully to make sure each app has enough headroom. It consumes considerably more RAM than Etcd and Consul.
Thirdly, I think it's fair to say that ZK is showing its age. It's notorious for being hard to manage (see the other comments in this thread), with a fairly old design (based on the now-ancient Google Chubby paper) that, while resilient, is also less flexible than some other competing designs.