No stars are expected with masses greater than 150 times our sun because?

All stars are in a balance between gravity crushing them in, and the pressure caused by nuclear fusion at the star's core. And the bigger the star, the greater the amount of energy generated.

At some point, the pressure of the stellar fusion is SO powerful that the outer layers of the star are blasted off into space. That point, we believe, is at about 150 solar masses.