This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Because that's just how those classes were designed. There's really no special reason.
You should always use HashMap and not Hashtable. Hashtable is a legacy collection class, which has more or less been replaced by HashMap since Java 1.2 (when the Collections Framework was added to Java).