Because the whole reason they introduced payed cosmetics was so they could create content like this for free right? Also locking content like this behind money in any game is bad, but in a game that is as stagnant and stale as vermintide is honestly just a poor move.
What we're trying to avoid is shipping paid content that splits players (see WOM). Choosing to use the Grail Knight doesn't restrict you from any lobbies or others from your lobby (unless they happen to be playing the Knight as well, as it works with Vanilla careers as well). These kinds of DLC will hopefully allow us to continue providing free content that would otherwise split the player base if paid for.