Opportunity attack with reach weapon against prone targets question

[jk7275]

I know according to raw if I attack someone prone more then 5 feet away with a reach weapon then I attack with disadvantage, i am unclear why this is.
Is it just to keep it simple or is there another reason?
Is it easier or harder using a halberd or spear to hit someone 6 to 10 feet if they are prone vs if they are standing up ?

[Boci]

Likely ease of wording. The intention is to give advantage to melee attacks and not give disadvantage to ranged attack if they are adjacent. The easiest way to word this was to specify being within 5ft, reach weapons became a victim of brevity.