It takes time to build a good reputation and its understandable if gamblers are hesitant to try new sportsbook/casino because of the doubt that it might be a scam. So what you can do is to prove that your platform can be trustworthy by providing a good service when it comes to security, withdrawal and online support. These are just some of the important factors that gamblers consider when they're playing. Offering bonus to try your sportsbook can also attract the gamblers and to experience playing on your site without requiring them to deposit first.
Usually new sportsbook try get trust from costumer or gambler by advertising and make promoting trough signature campaign on Bitcointalk forum, but usually have new sportsbook always give sign up bonus when early gambling platform launch. But most effective with new sportsbook get positive feedback from gambler how their website keep running well although have many people access and get withdraw process instant.
Running signature campaign has nothing to do with trust/reputation, those sites who think that signature campaign will give them good reputation are absolutely wrong.
Very clear example is 1xbit, they are running signature campaign but their reputation is really bad in this forum because they have so many problems.
Signature campaign is just a way to advertise the site, while trust/reputation is related to its overall services/product and it will be given by players.
Running a gambling platform in the forum that is already running for a long time is already build a good reputation because they listen to the communities recommendation and suggestions which is ideal i saw the improvements of their platform too before they don't have any sportsbook just a casual game of slots and tabletop games but right now due to players demand it is now integrated to their platform, of course, some of the members and users have the different biases in terms of their personal preferences such as security, and bonus.