Can you explain this? Wouldn't it be more expensive to rent a place plus a bunch of tables than to use an existing pool room?
The casinos are willing to provide a lot of stuff for free or near free (and sometimes even kick in added money for the tournament too), including the ballroom/s where the tournament is held among other things. It is worth it to them because in exchange they get lots of people in their casino renting rooms, gambling, eating and drinking, etc and they come out ahead of where they would have otherwise been had that pool tournament not been there those days.
So it doesn't necessarily cost the promoter more and may even be less (on occasion the casino is essentially on net paying the promoter to bring the tournament there). There are other benefits to the promoter too like having more space to be able to have paying vendors, and the fact that more out of town or out of state players are likely to attend if it is held at a destination location like Las Vegas, or in a casino, as opposed to Billy Bob's pool room in Timbuktu. People are more likely to travel if they can easily add some nice pleasure activities on top of the pool playing.
Usually the promoter has to guarantee certain numbers of room rentals and possibly other things and there is a penalty if they fail to meet them so while it can often be a great deal for the promoter it can come with some serious risk too if you don't get a good turnout. I am way over simplifying and every deal is very different but a deal might look something like this. Promoter gets from the casino the ballroom/s free for the event for the week, gets hotel rooms comped for tournament staff, water to be provided in the tournament area for all, discounted room rates for players and spectators, maybe even $5000 added money for the tournament, and other odds and ends. The casino gets guaranteed from the promoter by contract 1,000 rented room nights, and a 2% increase in food and beverage sales and gambling revenue over what it would have otherwise been on those same days. If the promoter falls short of meeting those numbers then they owe the casino say a $25,000 fee. It could also be structured where the promoter gets paid a bonus if they far exceed the rented room nights that were contracted.
So the answer to your question is that no it does not necessarily cost more to use a casino and is actually a better money making opportunity in many ways, but just like you can make more money by having it in a casino if all goes well, you can also lose more money if it doesn't. So you risk more to gain more, but you don't have a lot of choice because from an economic standpoint there just isn't enough money to be made by a promoter holding a big national event in a pool room to make it worth all the work that goes into it.