Casinos prefer linking no deposit bonuses to games that have lower returns to players (RTP). In general, table games have the highest RTP – it can be as high as 99% in blackjack. Allowing players to use bonuses on these games increases the risk of financial loss for a casino.
But video slots are much safer for casinos, as these games can have RTP that ranges between 85% and 96%. So most online casino no deposit bonuses are linked to slots, and not table games like blackjack.