Its not that you can't have special characters, but the way Larian handles them is clumsy.

Lets compare them to the characters in Pathfinder (in spoilers in case people haven't played it)

- Amiri

Normal barbarian out to prove herself by fighting stuff. Nothing special

Would work as Origin Character: Yes

- Ekundayo

Ranger out for revenge

Would work as Origin Character: No (met later in the game)

- Harrim

Cleric who turned away from his racial god to the god of the end times out of disappointment. Is blessed by his racial god to be some form of ultimate destructor of dwarven craft so that he can destroy unneeded things.

Would work as Origin Character: Yes

- Jaethal

Elf who was murdered, but raised as an undead by a god whom she now serves

Would work as Origin Character: Yes

- Jubilost

Famed writer and cartographer making a stop in your country now that it is being civilized to update his maps and writing articles about it

Would work as Origin Character: No (met later)

- Kalikke

Tiefling, cursed to constantly switch places with her sister, destined to never meet her again

Would work as Origin Character: Maybe, see spoiler

- Linzi

Bard wanting to have first hand experience of an adventure to make her writing authentic.

Would work as Origin Character: No (special event later on)

Nok-Nok

Comic relief goblin who thinks hes a hero

Would work as Origin Character: No (met later)

Octavia

Escaped slave. Normal half-elf trained in magic

Would work as Origin Character: No (met later)

Regongar


Escaped slave. Half-Orc with a bit of dragon blood

Would work as Origin Character: No (met later)

Tristian

Poses as a human, actually an angel who descended to fight evil which is found out much later. Vanishes for part of the story

Would work as Origin Character: No (met later)

Valerie

Fighter who left a paladin order because she was fed up by them

Would work as Origin Character: Yes

As you can see, it also has some special characters, but that is slowly discovered over the game. Most of the characters are normal and more or less average people. Also many of them are introduced later in the game instead of being available from the start which is a must with origin characters.