It’s simply because of the way the games are designed. Games with low house edges are often those that are synonymous with the Bitcoin gambling world, such as dice, while those with higher house edges are usually the older, more traditional games, such as roulette and blackjack – games that can’t be altered too much, otherwise the overall gameplay will change beyond recognition. You should find that the majority of Bitcoin games have house edges of 1% or lower though, meaning that you always have a good chance of winning.