Why Does The Parade Scoring System Work The Way It Does?
Obviously, I don't know Naoki Homma's reasoning for his points system, but probably the answer to your question is that turning the points around would drastically disbalance the game.  The main reason is this:

A positive point for you is like giving every other player a negative point

When playing with multiple players in a game, one must be very careful about giving one player an advantage because it's the same as giving all other players a disadvantage (and visa-versa).  This bonus is powerful enough so that a single extra card (making 6 cards) can make a huge point swing:  -10 -9 -8 -7 -6 = -40 points to +6 * 3 (in a 3 player game) = 18 points.  So one card makes the difference of 58 relative points in the game.  The other way, it turns -40 into -6, which is still huge (34 points), but nearly half as big.