To reiterate a bit on what hardwaresofton has already said, yes you could use Cassandra. However, Cassandra like ZooKeeper is just a building block. It doesn't actually provide service discovery, health checking, etc. You could use it to build all of those, but it doesn't provide it.

We compare Consul to ZooKeeper here, but much of that applies to Cassandra as well: http://www.consul.io/intro/vs/zookeeper.html

Internally, Consul could also use something like Cassandra to store the data. However we use LMDB which is an embedded database to avoid an expensive context switch out of the process to serve requests with lower latency and higher throughput.