A Jotun probably has crazy high Holmgang stats. So, they will probably whoop anyone in Holmgang except a God.

A Jotun knows who the other Jotun are so they don't have to worry about accidently challenging one of their own.

A Jotun probably doesn't have to worry about accidently Holmganging a God since Gods are less likely to challenge. Unlike Jotun, Gods don't want to kill townies.

In Midgard 1, Stig's Holmgang didn't really reveal that he was Jotun.

So if a Jotun can keep their identity hidden, and usually win in Holmgangs, then it is in their interest to challege every turn.

GH's "It is there so I will use it and to hell with what you think" attitude fits this perfectly.