Why is it that JEditorPane cannot display some of the HTML controls other than text? How can I display buttons and other controls?

The JEditorPane is not a full blown implementation of HTML browser. It does not support an HTTP forms interface. However you can add components like JButton to the JEditorPane using the following method -

SimpleAttributeSet sas = new SimpleAttributeSet();

StyleConstants.setComponent(sas, new JButton("Hello"));

editorPane.getDocument().insertString(
editorPane.getDocument().getLength(), "", sas);