I guess it's because of the business rivalry between fellow gambling platforms who want to see their rivals tampered with on their competitor's sites.
The DDOS attack will temporarily attack the gambling site but if the gambling site has a qualified security team, the DDOS attack will not affect the site.
We will never know who launched the DDOS attack and can only guess.
That is why a business on the internet must have a security team who is an expert in their field to deal with DDOS attacks that will come to their site.