A couple of witnesses have made comments that led me to consider that it could be true that Arthur has an intellectual disability (the witnesses, MM and someone else, used more disparaging terms). If that is the case, it may explain why he has not been called as a witness. He would lack credibility and perhaps not meet the criteria for being sworn in.
Of course, MM and others may just have been insulting, but in general, a person with real intellectual limitations is not usually called as a witness, especially if he or she is a peripheral character. The SCC has ruled however that people with intellectual disabilities can not be excluded when their testimony (for example, re abuse) is vital to the interests of justice.