I'd say it is to do with class maturity and support.
Some classes released in earlier Java versions have been superseded by classes that make their existence obsolete. In some cases it is possible to mark the older immmature versions as 'deprecated' and announce when they will no longer be supported.
In some special cases this is not possible, and the class is considered legacy ie 'should not be used from now on but will not be removed'. Some of the pre-collections classes have been retrofitted to support the Collections interfaces, but the pre-collection Java 1.1 version has special meaning to some Applet, embedded Java and micro-Java cases and therefore needs these classes to exist. So Vector and Hashtable are considered legacy but are unlikely to be removed.