If the project has no budget, no big investors, it is unlikely they can attract the community on such sites.
While SUI appears to have big investors based on the number of projects that is already deployed on the platform but there are other projects on Crew3, now called Zealy, that has big investors but small community.
Zealy got paid for listing projects on their platform, it is now left for the project to introduce tasks and reward that will attract participants. Why SuiSwap for instance have a very large community is because of the promise to airdrop Sui to all the participants based on their XP points that will eventually be converted to Sui points. Other projects can decide to reward only the top 20, top 50, top 100, or top 1000. I won't want to join such campaign if I have no chance of getting a reward.