Okay, here is our resident Merc insider's reasoning for the Pirellis

The Michelin is a new tire development, and may not be in customer production yet, and MB simply may not have a sufficient supply yet, could be a reason for the limited availability. Even the press pictures included with the article showed Pirelli and not the new production Michelin tires. The GT R is expected to arrive in the showrooms in March 2017, but customer deliveries may only start later, at the end of May or so. By then the new Michelin Pilot Sport Cup 2 MO tires in the dimension 275/35 ZR 19 front and 325/30 ZR 20 at the rear should be readily available. 

PS. The GT R may be sold out already in Germany for next year (2017).