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Originally javax was intended to be for extensions, and sometimes things would be promoted out of javax into java.
One issue was Netscape (and probably IE) limiting classes that could be in the java package.
When Swing was set to "graduate" to java from javax there was sort of a mini-blow up because people realized that they would have to modify all of their imports. Given that backwards compatibility is one of the primary goals of Java they changed their mind.
At that point in time, at least for the community (maybe not for Sun) the whole point of javax was lost. So now we have some things in javax that probably should be in java... but aside from the people that chose the package names I don't know if anyone can figure out what the rationale is on a case-by-case basis.