The reason why there is no math prize:

1, because an influential mathematics prize was created at that time, called Scandinavian Prize.

Nobel wants to reward people for their "practical" inventions. He believes that mathematics does not belong to the category of applied science and will not bring any benefits to people.

The reason is that the girl that Nobel always liked (used to be his girlfriend) married Mittag Leffler, who was especially famous in Sweden later. If Nobel sets up a math prize, the first winner will be his rival in love! This is what no spoony person wants, knowing how much Nobel likes that ex-girlfriend. It can be seen from his lifelong unmarried.

Moreover, the Nobel Prize in Economics is not a Nobel original prize, but a later added project.

Extended data:

10 February 10, the famous Swedish chemist alfred nobel died. In his will, he proposed to use part of his legacy ($9.2 million) as a fund, with interest divided into five categories: physics, chemistry, physiology or medicine, literature and peace, and awarded them to scholars all over the world who have made great contributions to mankind in these fields.

Baidu Encyclopedia-Nobel Prize