There are several reasons. Philosophically we don't think that discounting artwork is great for brand and given art is a Giffen good, so the more expensive the price the higher the received value. That being said there are different motivations and discounts are a good way to convert a first time patron.
The second reason is that because we notify your followers of the sale, we dont want to bombard them with notifications.
We will continue to be experimenting with this, so please let us know what you think.