Agreed - but it seems that this is an authoritative-only name server, which means it shouldn't be susceptible to cache poisoning, which is where the most awful DNS server security vulnerabilities have been.
Now that we have them, couldn't some core code be shared between all these projects and receive more scrutiny than it receives now? For example the code that parses incoming packets or generates replies could easily be shared (in theory).
Agreed that code sharing would be nice, and more important than ever since the rise of small servers with specific focus.
There are also advantages to having independent implementations for such crucial software. A fatal flaw that no one caught in one implementation may not take out the whole system.
Smaller problems require a lot of time, attention and testing to be found. To have a single code base can do wonder for this kind of smaller, but often fatal, problems.