They won't disclose the reason for it but I'll take a guess that it's about the number of customers that they're having. Because if there's decent number of customers that contributes to their operation and profit, they might just continue it.
I'm thinking the same, it is reasonable to close the site down once the owner feel that the business is no longer give them good profit.
The competition in this gambling industry is so tough, casino needs to compete with other existing casinos by offering something different/better if the casino want to compete then survive.
In this case, onehash seems to be lack of offers, promos, as well as advertisement if we compare it to other leading crypto casinos.
Perhaps they will come back with a new project in the future after closing this one down, who knows...?