Why no? You seriously need to ask? Pence was an integral part of the campaign, not some minor, temporary volunteer. If there really was Trump campaign collusion with Russia to somehow affect the election, Pence and/or his top aides would know about it. So why has Mueller left Pence alone? Potential answers range from the possibility that Pence can be controlled all the way to Pence, in many ways, is the opposite of Trump. He’s a well-liked but bland former Member of Congress and a governor with a squeaky clean image that makes him untouchable. So although we don’t know why, the question is valid.
You may be right. I’m only throwing out the possibility that there really is nothing for Pence to say, and no reason for Mueller to be interested in him. It may be a conspiracy and it may be nothing.
It’s probably something somewhere in between.
A question that keeps needling me is what about Wikileaks? If Trump colluded isn’t Wikileaks a co-conspirator that lied about its source being a non-state actor? Why hasn’t Mueller requested an interview with Assange?