Yeah, marketing is important, but not all casinos have a vast amount of budget for marketing. Moreover, I think they are not satisfied yet with the MVP (current product) and want to improve the site first before adding marketing programs. IMO that's why MintDice decided to scale back its signature campaign to be more efficient in this building process.
No matter what level you are with your product, marketing is a must, there are free marketing methods as well or even very cheap ones, if you want to run a casino and get known you have to have a marketing budget, if you start with zero then you will not go big and mintdice is not like that. They have been doing that signature deal for a looong time now, they came out of the gates with guns blazing and they have turned that into customers who lost money and kept going.
So, I assume they have a marketing budget and they have been using it on signatures. Maybe they can try other stuff as well but I feel like as long as they keep going with the signature and just give it time, there will be more and more people eventually. It is just the way it is, adoption requires time.