Answer:
because matter becomes more compact
Step-by-step explanation:
The magnitude of the gravitational field is given by:

where
is the universal gravitational constant,
is the mass of the star and
is the radius of the star.
We can see by this equation that if the radius becomes smaller the gravitational field will be bigger, because they are inversely proportional.
So when the star is shrinking, the radius of the star gets smaller than it was originally, and the mass will stay the same. So the result is that the gravitational field intensity will increase.
In summary:
the gravitational field intensity increase on the surface of a shrinking star because matter becomes more compact (the radius becomes smaller)