There's some additional info in the link that I provided:

Forking classes from current classes or additional talent trees. But no, because the character classes wouldn't feel distinct. Would all be watered down.
Multi-classing -- a secondary class or a hero profession. For example, a warrior would become a deathknight. Would water down the different character classes. How is a warrior death knight different from a warrior death knight, etc.
What is a mage deathknight or druid deathknight -- wouldn't feel right.
Class morphing -- the character class would stop being the old class and then do the quest and become a deathknight.
Players would feel they were losing existing characters. What if you don't like the new one. So they decided to unlock instead of create new.
Looks like they tried to create a subclassing/multiclassing system similar to the noblesse classes of lineage 2, but weren't able to get the design right.