An idea introduced to a population almost always fades away.
Send 1,000 people a coupon, and perhaps 20 use it. To get more usage, you either need to ping the audience again or find a new group of people.
This explains why marketers are always in search of new people to reach, and also insist on frequency of messaging–it maximizes the percentage of the group that is reached and minimizes the fade of the idea.
There’s an important exception to the rule of fading ideas, though.