I suppose it doesn't HAVE to be that way, but Larian wants to make use out of their new Source Point system and source skills, and those only are useful for Sourcerers.
That's certainly a point, as in D:OS all companions had the potential to learn every skill. OTOH, you'd rarely have 4 Marksmen, Hydrosophists or Scoundrels in your party. So, barring extra special sourcerer mechanics, not the whole party would end up becoming master sourcerers. Instead you'd strive to have a broad array of skills at your disposal, ideally by building upon the inherent skills of a companion you like or by picking your companion based on the skills that complement your party. So a potential companion would be useful even if unable to perform sourcery.
The main characters also have a story goal to get to Black Ring territory, and non-Sourcerers wouldn't share that goal.
I can't really see any good reason for someone who is not a Sourcerer and hates Sourcerers to be hanging out with a party of Sourcerers who are heading for the territory of the Cult of Mustache-Twirling Evilness (you must be this evil to apply).
What reasons do the other prisoners have to head there? Why shouldn't they leave the player once they made it to safety and
return to their loved ones, or go into hiding, or start their own Evil Empire in a galaxy far, far away?
I say they'd have the exact same reasons that a non-Sourcerer would have: either it's something personal or they somehow feel attached or indebted to the player. The writers shouldn't have problems to come up with something that's plausible.
Besides, I doubt that players would walk up to a potential companion and go something like "Hey there, I'm this escaped Sourcerer on my way to the Cult of Mustache-Twirling Evilness and I want you for my party!". Instead it may start pretty innocently, through a random conversation or a small side quest perhaps, and by the time it is clear what you are and where you are going, it's too late to turn back.
