Why are commas, rather than dashes or parentheses, used in this sentence?

A.
Commas are being used to set off an appositive.
B.
Commas are being used to set off a clause using which, who, whom, or whose.
C.
Commas are being used to set off a longer or less-related parenthetical aside.
D.
Commas are being used to set off a brief, closely-related aside.