They’ve been doing stuff like this (separate pics photoshopped into groups, changing outfit colours, etc) since at least 2009. I remember the 90210 and Melrose Place reboot cast pics were done exactly the same way.
I think one of the reasons also might relate to actor vanity. In a traditional group pic, there’s always the risk someone might not like the way they look, etc. This way, they can take a few shots, photoshop them to within an inch of their lives, slap them together, and everyone looks “perfect”. Doing it this way also makes it easier to remove cast (if they leave the show), without having to do another photo shoot. Especially important in a show such as this.