Looking at this page that aggregates the current NoSQL landscape, one can see that the majority of these projects are written in Java.
Databases are complex systems software dealing with the file system, and so C/C++ would be a better choice than Java for this. (that's my thinking which might be flawed)
Secondly, databases deal with transferring large amounts of data from disk to RAM -- which they call a working set. The JVM takes non-trivial amount of RAM for it's own purpose -- so it would be more efficient to use a platform that leaves lots of memory for data instead of hogging it for its own operations.
The major relational databases are ALL written in C/C++
MySQL C, C++ Oracle Assembler, C, C++ SQL Server C++ PostgreSQL C SQLite C
So what makes Java so popular in NoSQL world.