I think It's because it is the most appropriate way of classifying everything by grouping of five.
Grouping by a singlefold classification would be "all" as is defined in sabba sutta. It is from that a matter of inference.
In that you infer elements that are not included in the previously inferred one's until it is no longer possible to name an element that isn't included in the five fold classification.
The Sabba Sutta also gives a twelvefold classification.
"Monks, I will teach you the All. Listen & pay close attention. I will speak."
"As you say, lord," the monks responded.
The Blessed One said, "What is the All? Simply the eye & forms, ear & sounds, nose & aromas, tongue & flavors, body & tactile sensations, intellect & ideas. This, monks, is called the All.  Anyone who would say, 'Repudiating this All, I will describe another,' if questioned on what exactly might be the grounds for his statement, would be unable to explain, and furthermore, would be put to grief. Why? Because it lies beyond range."https://www.accesstoinsight.org/tipitaka/sn/sn35/sn35.023.than.html
Here one can't name an inferrable element #13 that is rightly inferred and isn't included.
It is the same principle but the grouping is by five for khanda.
The 5fold classification is needed to delineate rupa, arupa and the genesis of those. The five are otherwise rupa & any other of the two because the 4 are conjoined but need delineation to explain genesis and factors of the rupa and the arupa states.