Why do the figures assigned as personas to the main characters always get this sort of high-tech, Cyberpunk look to them? Rather, why are they the only personae in their respective games to get this treatment? It just looks out of place when you as the player character swap out your initial persona for another one and it's of a more traditional design (well, traditional as far as mythical creatures and figures appearing int Atlus games are concerned) next to all these futuristic-looking entities based on mythical figures that your allies have.
I think it's just to show those particular Personas are unique to each of those characters. Compare Yukari's Isis with P4's Isis which does use the SMT Kaneko design. Also, using the latter for the MC's stock of Personas only really happened starting Persona 3. In Persona 1 and 2, most of the available Personas have art unique to the series (a few exceptions like Cerberus and Innocent Sin's Nezha/Nata Taishi). For example, this was how Shiva was designed in Persona 1 and 2, and he wasn't a signature Persona either◊.