I have multiple reasons:
1. Time - to create a new card (think of a good card idea, find a nice picture, balance, implement, test) takes an hour in average. Concepts section does help, since nice ideas and card pictures can be found there, but still it takes time.
2. Balance - I try to add about 20 cards in every card update. This way, the new cards won't imbalance the game very much or at least the probability is low.
3. Try to look at it from the point of view of other players. Some complained that card updates are too often and they don't have time to adapt. Each player has it's own pace, so I must keep it somewhere between. I cannot focus on hardcore gamers only.
2 and 3 may be questionable, but 1 isn't ;-)