Since in trading you have the option to lower your risk and improve you chances of profiting unlike in gambling once you roll the dice there is no turning back. And on gambling there is no way you can lower your risk or improve your chances of winning, it is all but fixed chances.
Sure you can lower risk in dice. Roll dice on multiplier 100X - high risk big reward, roll dice on multiplier 1.01 - low risk - small reward.
Trading - buy 1 coin for 1bitcoin, whale come and dump his bag, coin loses 50% of it's value - and you lose half of your money. Both are very risky and if you don't know what you are doing trading can become big gambling as well.