well the mp and collab rule is on bc the replay have to show your skills only
if it was mp it wont reach the standars of an sp replay anyway
a collab will show 2 people skills and as you know 2 minds are better than 1 bc if someone didn't find a good solution for a part the other guy can get it and at the end you can do a godlike shit which is much harder if you do it alone.
so yeah sp is better than mp
collabs make shits much easyer.
atleast thats why i think they put this rules