The rear wing, due to FIA regulations, has very few areas that can be altered, making changes difficult and, most importantly... expensive.
A new rear wing costs at least 1 million US Dollars from design to manufacture.
For the Mercedes team, which already has a budget cap, this is not a small amount, especially since half of the money for this season's car upgrades has to be used to develop next season's car.
In comparison, the front wing is much cheaper, and most of the adjustments to the front wing involve subtraction, with very few additions.
Crucially, the reduced section at the back corresponds precisely to the entrance of the Venturi channel of the Mercedes car.
After reduction, the airflow throughput of the Venturi channel can be increased, and by then, whether it's the airflow inside the floor, or the vortex quality, speed, and sealing degree generated by the vortex generator, will all be higher.
