Online gambling is a hard business to be in as daily new casinos are popping up and some try to offer new features/games/giveaways that can't be found on other sites. So it is one thing to keep attracting new costumers but the hardest part is making sure the people return to the site and keep playing. As you say, Roobet is a pretty big name already and they keep doing stuff (like the art contests) to stay relevant so that is nice.
I think it's also part of their marketing so it's important to have a marketing team that makes everything look fun so that many users are happy with Roobet and always come back to play there, it's true that maintaining consistency is complicated especially now that many gambling sites are getting busier which of course will making the competition even hotter, but Roobet continues to consistently run campaigns that they still maintain, including contests.I am not a very active user on Roobet but they are very active also on social media often giving out free money to their active users moreover interacting with the community is also the most important thing to do, so far Roobet has been very consistent with their contests on this forum and also other campaigns in this forum, it seems they have loyal users who are still playing there to this day, that's why their name is so big in the forum and out there.
Marketing play a big part there and Roobet manage to sustain this that's why we see them earn success because they are actively doing some good marketing that attract a lot of people. We can also add up their casino performance which play huge role for their success and also for sure we can see them succeed for more years since they are already well establish and people already know that they can trust Roobet compare to any other casino.
Although they decrease the size of their campaign participants maybe to test out if their casino still sustainable even with that numbers. But maybe once they need more exposure they increase the number of participants and also add more contest since they are very well known to that.