The Wealth of Nations was published in 1776, 57 years before Alfred Nobel was born. He was aware of the field of economics and didn't think it was worthy of a prize.
Fields such as mathematics and philosophy were also around in Nobel's time and he didn't think they were worthy of a prize either. The difference is those fields aren't associated with an organization that literally prints money to buy their way in.
> He was aware of the field of economics and didn't think it was worthy of a prize.
I doubt you have evidence of this, beyond the simple fact that he didn't personally establish a sixth prize. If such evidence existed, I think it's unlikely that the Nobel Foundation would've agreed to administer the prize in the first place.
That said, at this point, I'm not sure what the difference would be anyway. The Nobel prizes (including the memorial one) have become a globally admired celebration of human achievement, the personal beliefs and shortcomings of the 19th century arms dealer who established the prizes notwithstanding.
Fields such as mathematics and philosophy were also around in Nobel's time and he didn't think they were worthy of a prize either. The difference is those fields aren't associated with an organization that literally prints money to buy their way in.