The point of the prosecution was to be a warning to others that might have been thinking about not appearing. Bannon offered to appear once it became obvious th

The point of the prosecution was to be a warning to others that might have been thinking about not appearing. Bannon offered to appear once it became obvious that he would be convicted but the committee refused that offer. They obviously had decided that value of the conviction was greater than any value of him testifying at that point. Once convicted any leverage to compel him appear was lost.