I bet the 'commercial' viewpoint here is that it takes time to design a separate weapon for each upgrade which is what is needed if we want a more powerful one per upgrade. This is what used to happen. I dont know if this will work but perhaps 6 x this unique weapon is on average more powerful than 2 x weapon e.g. average for 6 x weapon is 50k att while while 2 x weapon is 24k att.

The other consideration is we have to ensure the right stats are being applied so it could get messy from Gree's pov. Alternatively why cant they use existing weapons handed out in previous events.