Can you explain why lasik can only work on the eyes up to a certain % of vision loss with astigmatism?

Cornea limitation. Depends on the cornea shape, thickness, and how much and where tissue can be removed in order to counterbalance astigmatism. If too much tissue is removed it can cause problems and ectasia and if the wrong tissue is removed, it can create more refractive error.
LASIK astigmatism . Laser vision can correct the regular astigmatism of the vast majority of patients. If the astigmatism is irregular in nature, then laser can not be performed.
Corneal shape. Astigmatism can best be thought of as a spoon or football-shaped cornea. Lasik reshapes the cornea to make it round and alters the degree of roundness to adjust focus. Lasik accomplishes this by removing corneal tissue and we have to operate within safe parameters leaving enough cornea behind. Astigmatism can present problems with this.