Underdog's are usually more relatable to people than the untouchable champ who already has his followers and reputation. The champ is always this golden shining presence while the underdog has something to prove.
It can be a few things. People think the underdog made it up to this point because they worked harder than anyone else and struggled doing it so they deserve their spot at the top.
People may be jealous or tired of the current top position holder and want to see him dethroned. They want to see change or see something exciting.
It invokes a sense of satisfaction and a spike of emotion when the underdog finally gets his day.