As I understand it (as a regular on the ZeroMQ and Crossroads mailing lists), the reasons are as follows:

1) Trademark issues - the original developers of ZMQ have been prohibited from using the ZeroMQ trademark and, thus, from making official releases.

2) The new ZeroMQ contribution policy[1], in which pull requests are merged without review and then later reverted/enhanced if necessary, is seen as incompatible with the goals of some of the lead developers. These developers prefer a more rigorous review policy.

There are more reasons, but from what I can tell, those are the main ones that haven't fully been covered here.

[1]: http://www.zeromq.org/docs:contributing (see "Philosophy").