That is probably one of the most expensive parts of running a casino, specially one which is not well known and is trying to get some attention among a sea of competition.
Advertisement campaigns are expensive in general and anyone wishing to start a casino should not only consider to hire people skilled in coding, security and art (in order to keep their games and seek for bugs) but also a PR/advertisement department would be necessary, to keep attracting new gamblers in.
It is possible to have success with a small ad budget, but it would require the casino to be so good and so enjoyable that people would be willing to spread the word by their own in a massive way, that is not so common to see nowadays. Sadly, it is easier for people to spread the word about a disgusting ponzi scheme than a good casino which is new.