aside from the fact that gambling advertisement might be banned for certain countries across the world its also probably due to the fact that advertising on TV will not give the gambling sites themselves targeted audience.
I think TV are mostly just some people try to enjoy shows not betting, quite opposite if we are talking sports, its quite effective thats why in football matches the gambling ads are everywhere because its targeted audiences I think because surely in every match some people want to gamble.
On the contrary I think TV advertisement is the biggest for gambling companies. TV has wider converage and easier to access. Most gambling companies would like to throw up their adverts on the screen, some adverts just go pass the screen while program is on while some pay for full time for their adverts to be showcased from time to time, daily or weekly.
So if a gambling company is ban from running adverts on TV then it is a big disadvantage from my observation