Eryka would be cool as a companion, but I like Viv too. (sigh) why can't we have both? Oh right Savin...
Companions are restricted to the staff, for a matter of compromise to the project and reliability.
You write a companion, you have to be constatly adding content for them, reacting to the missions and events of the world, helping the PC deal with truoubles and challenges on quests, creating sets for them, varying degrees of character development...
Writing a companion is a huge responsibility, one that you have to fulfill for years.
Eryka is not a companion because her writer, Franks, didn't make one. Whether the reason was because they didn't want to or lack of time with real life demands taking priority, it is what it is.