Planes have many features that contribute to their performance. And that variation in performance creates a vast array of opportunities to change the outcome of an engagement. So if a new plane like the P-63 is added it's combination of speed, acceleration, roll rate, turn rates, climb, gun package, visibility, dive, ordinance packages, range, toughness, stall characteristics, all which vary with altitude create a whole new and unique dimension to game play as people learn to fly it and fly against it. This uniqueness increases exponentially when multiple planes of various types all engage each other.
While tanks have multiple attributes that affect performance, they are much fewer. Muzzle velocity, AP capability, Optics of the sight, armor protection, Turret speed, and tank speed. But most AH tank engagements are less than 2000 yrds in wooded, hilly areas. In this peek-a-boo environment the muzzle velocities of most tanks is sufficient for one shot kills. The variations in speed, armor, range, AP, optics are all neutralized. The battle comes down to who lands the first shot, where it's much more about the strategy and skill of the driver with respect to being sneaky. What percent of tank deaths are by a tank they didn't see, or saw too late? 75%? It's a very rare engagement in AH where tanks are exploiting advantages in speed, turret mobility, firepower and toughness out in the open range, over long distances to maneuver around each other slugging it out to determine the winner.
So unfortunately a new tank with slight variation on those attributes doesn't effect game play very much.
