A lot of Anarchs have generically powerful, if situational abilities. See Edward Kim, Whizztard, Quetzal, Omar, Reina, Null ect. This means that they can all play a "good-stuff" deck, and compliment what they feel they're lacking with a switch of ID.
Contrast this with a lot of the Shapers out there, like Haley, Professor, Rielle "Kit", Ele "Smoke", or Criminals like Armand "Geist", Ken "Express", Iain, and even to an extent Leela. All of these have abilities that lend themselves toward deckbuilding in a cetrain direction, thus making them more interchangeable.
Not helping the matter is that Anarchs in general have a HUGE slew of generically powerful cards, decent or better in-faction breaker options, and pretty powerful economy tools, leading to more similar decklists.