If they put in Sophia they'd be losing a lot of damage
Thus they wouldn't have achieved such a fast win time.
See those big 89'000+ damage per hit numbers - those are Peppita's
The best reason to use Sophia is ... to have a mascot
The strongest and most broken in the game are Maria with Radiation Bots and Peppita with Dream Hammer and Fayt with Ethereal Blast or Cliff with Max Shockwave.
Personally I'm starting to like Fayt more than Cliff now though - Max Shockwave only seems to get like four or five hits for me - what's good high damage mods if you can't hit a bunch of times
Unfortunately characters like Sophia and Nel lose out damage wise on the super bosses since their best skills are elemental and the end super bosses resist elemental damage =/