TOKYO: Due to Death Knight’s ability to start the game at Level 55, only way to play as this class was to have a separate character of Level 55 or above; which was a very big deal back then.
Now, due to the presence of Level 90 character boost purchase, Blizzard has realized that this requirement is no longer necessary; which is why they have decided to remove it.
Community Manager Nethaera in a blog post stated that Death Knight will continue to start at the Level 55 but there will be no requirement of having a separate character at the same or above level.
The plan right now is to remove the limitation sometime next week, at which point players will be able to start a new Death Knight (which still begins at level 55) without first having to reach level 55 on another character.
Not a lot of players now want to grind their way to Level 55 which is why they just pay for the Level 90 boost, saving themselves a whole lot of trouble and time.