Why Mostly Betting Platforms promote their Website through Signature Compaign?
One possible reason is strict advertising regulation by government and rule by advertising company. For example, Google require you to get their certification before you can put ads about online gambling[1]. And many county such as US forbid casino to advertise itself on TV[2] and other medium which viewed by all kinds of people.
Cause gambling is not for everyone.
Trading and mixing (another service which utilize signature campaign for advertising) is also not for everyone.
[1]
https://support.google.com/adspolicy/answer/6018017?hl=en[2]
https://advertisingrow.com/advertising-magazine/can-casinos-advertise-on-tv/