Why is gravitational force in the nucleus so small?

Because it's mass is very negligible and gravitational force is directly prolortional to the mass of a body.

It is because of two factors:

1) Their mass is so small and as gravitational force is directly proportional to mass, it is also small in magnitude

2) The distance between nucleus and other particle (electron) is very high as compared to their own size, and as distance is indirectly proportional to each other, gravitational force is very low

Hope this helps!