Question

why can't we Find determinant for rectangular matrix

Answer 1

The determinant of a matrix is a scalar property of that matrix, which can be thought of physically as the volume enclosed by the row vectors of the matrix. Only square matrices have determinants. 

Answer 2

Determinant of matrix is calculated only for square matrices. For non-square matrices, there's no determinant value. Same is the case for Inverse of matrix. It's quite simple: you don't.