Because it allows the monarch to say lol fuck that and change the system as necessary in different situations. Also vesting power in the monarch. It's a good thing... I do truly fail to see what problems you two honestly have.
I oppose it because it makes the monarch practically untouchable. For example he could change procedure to force cases against him to not be accepted, and not only that but the Jarl could also be dismissed for accepting a case against the monarch.
Essentially you are removing all accountability from an already less-than accountable person/position.
You also open up the path that puts limits on the legislation that the storting can produce, in which is already rather limited by saying that Judicial Procedures are not a form of law(which they currently are), but are instead executive powers, thus meaning that the Storting can't fix any errors, and the Monarch can refuse to fix them..
Now while I do realize that Wintermoot probably won't do this, but it does lead up a possibility for future Monarchs. While it may work at this moment, what about when wintermoot gives up Monarch? Someone could easily get it and do this.
We also have the issue with Sham-trails that could be forced by the Monarch(again not talking about Wintermoot in particular) that is designed to remove an unpopular person(or someone the Monarch doesn't like/has a low regional approval rating) thus also violating the Fundamental Laws, and since it's part of the executive, the people aren't really accountable, in any form, to the citizenry, but instead only accountable to the Monarch.
In reality this system is open for a LOT of abuse and creates an issue where, unlike previous systems which were at least somewhat accountable to the citizenry(our current system with has Storting members that could be removed, or the proposed system which cases could be brought against a judge should they have accepted a bribe) and moves it to the Monarch, where the Monarch influences all of the outcomes due to the fact that they(the Jarl) could be threatened with dismissal if they do something that the monarch dislikes in which the Monarch could effectively undo that.
Although if we are to do this, I would prefer it if the Jarl appointed was to be confirmed by the Storting, and any changes would have to be approved as well. Although keep in mind this would only apply to this Position and no others.
I would also support a bit more of a hybrid-system in which the Judicial is shared between the Monarch and the Storting collectively.