Well, I don't work there anymore, but my guess is that the D&D Encounters program has show WotC that if you provide a fun and fast game experience, people show up an play it. It also shows that those folks rarely spend money in the store that is hosting the event. My guess (through limited observation, nothing more scientific) is that 1 out of 4 people who participate in D&D Encounters regularly make purchases at the stores where they play these games.
1 out of 4 is better than having none of those people come into your store to buy anything. Also it is the same for Magic the Gathering. Also Magic the Gathering sets a tournament entry fee because it gives prizes for the tournament. This isn't something really done with D&D as you aren't playing a tournament, and stores wont really want to go through that hassle and space to run a D&D tournament with the time it take and table space as opposed to a CCG tournament that can generate more money due to quicker turnover time from the tournament. Also some smart stores have random booster drafts, where the players decide they want to draft, and the store takes a flat $12 per person for 3 booster pack which is less than MSRP, and even gives a booster pack or 2 as prizes on top, meaning they moved merchandise, even if it wasn't sold at full price. But that moving merchandise made them some return on their invested real quick as opposed to waiting for those 24+ booster packs to sell. Box of boosters sold respectably at $120, otherwise about $3.50 per pack, and you already got $96 of it back leaves only $24 left to get from that remaining 12 packs, and you just figure it comes out better than if you had sold the whole box at once.
D&D doesnt have anything like that and the fortune cards wont do that for the store to push D&D products to make any decent amount of money.
Now to the bolded portion. The reason RPGers don't spend money in the stores is because they really have nothing to offer them. Some stores have a little hot dog machine and will sell a dog for a $1, sodas for whatever they have gone up to know with insane inflation and the tax being shifted to the customer rather than the manufacturer. So compared to a hotdog you can make close to 100% profit from, what kind of profit margins will these fortune cards bring that can compare?
I would say 1 in 4 people that go into a Walmart don't buy anything. But Walmart doesn't have space to play games or anything save for video games to test. Doesn't seem to bother Walmart. So you have to have something that they will want or need to buy. Sitting around for a few hours playing an RPG means you will need snacks, drinks, and bathroom. You start charging to use a bathroom you likely to even lose CCG players. That leaves snacks and drinks. If you don't offer then then they will send someone somewhere else.
RPGs just don't have the pull in a store to bring in the money that CCGs did, and this was sen shortly after 3rd edition. There is nothing really to offer an RPG player to get them to pay to play in the store, unless you just rent the gaming space. Every TRUE needed component of the game is buy once, and not for a small price, save for those that collect dice; and who doesn't.
Making someone buy some new component to play a game that works fine without it for them, isn't really going to make them want to be at your store, or be in those events.
The D&D events really are to get butts in seats to make the store look well off and busy, and to hope those RPG players will cross over into other gaming lines if they haven't already to buy some of the things that sell quicker for a slightly better profit margin, like CCGs, minis, etc. I have seen people buy boosters of those Clix because they ran out of different types of D&D minis and needed something for one game.
Also when in a store selling CCGs, you can't help but see the CCG players, and not every D&D player wants to turn into that guy with the notebook of DDM cards in pages to trade with for X.
It takes so much time to play D&D face-to-face for many, being able to get together and do it with a fully prevent event means you can just get to playing, not have to spend time building some deck of cards in advance.
RPG players aren't as renewable customers as the CCG gamers since their games change constatly due to new sets.