It's not leftover morality laws; it's that monitoring commercial gambling to protect against exploitation of the poor is a really hard problem, one we haven't really solved. Some states choose to allow it despite this, and then deal with the consequences of increased poverty. Most don't. (And then they're forced to allow it in anyway by native on-res exceptions.)
This is also why such commercial gambling establishments as are legalized, are heavily regulated. For example, see the Nevada state laws on gaming fraud: https://www.leg.state.nv.us/NRS/NRS-465.html#NRS465Sec070
Those laws aren't there to stop people trying to cheat the house. They're there to stop the house from exploiting people by rigging the game.
And we all know no poor person in these states ever gambled their money away! Or, alternatively, they did gamble it away, but since gambling is illegal, they were also cheated mercilessly and have no recourse and no place to complain, since being poor and telling police you participated in illegal activity gets you an express ticket to jail.
Gambling is viewed as entertainment, both by the government and the people participating in it.
As entertaining as startup life can be, there's a very big difference between a risky investment and gambling for entertainment value.