- Cassandra has finally gathered a community behind it 
- they have identified good or common use cases
Secondly, the NoSQL world is quite wide. Cassandra is a column-oriented store (in the same category: BigTable, Hypertable, HBase), but we also have key-value stores, document stores, graph stores — see ,  and  for more details — so saying that it is winning the race is incorrect. So, at best it should be compared with the other column-oriented solution.
Thinking of HBase, we recently learnt  that is doing well too, that there are real-life production applications running on it, and that it has seen good performance improvements over the last couple of releases. And as far as I know there is a larger community behind it.
You should also check the HBase vs. Cassandra: NoSQL Battle! article to better understand how they compare and where they differ and also Cassandra Gets (Better) Documentation for some very good references.
- ☞ Is Cassandra winning the NoSQL race?
-  ☞ Looking to the future with Cassandra (Digg)
-  ☞ up and running with cassandra (Twitter)
-  Cassandra Gets (Better) Documentation
-  ☞ NoSQL Ecosystem
-  ☞ Quick Reference to Alternative data storages
-  Musings on NoSQL
-  NoSQL with HBase
- HBase vs. Cassandra: NoSQL Battle!