Because you should be able to find a good hash data structure for your language. You shouldn't be able to accidentally get a bad one. It should arguably be the first data structure you reach for.
You can even find this in the javadoc of the Vector class:
>As of the Java 2 platform v1.2, this class was retrofitted to implement the List interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Vector is synchronized. If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector.