What mtpelion said is right. The presumed amount of gems that would be sold with the discounts or offered bonuses would probably not offset the losses in revenue or not be good enough in the long run (when you put up sales or bonuses, as a company, you should always think about the long term effects because people will certainly demand more sales if you’re planning them as short term campaigns).
That only proves that enough people are buying enough gems at a rate that is regular enough for sales or bonuses to not be necessary or profitable.
Also, no matter how you think about it, a discount in gems or an offered bonus WOULD largely affect the rate of Gold:Gems and Gems:Gold as they are both entirely reliant on the supply and demand of the other.