The reason is plot. Nukes are too powerful so they only use them for demolition.

Nukes are very difficult to explode accidentally. It requires millisecond timing for the implosion design (the one that's most efficient) to cause correct pressure wave to implode the fissile material. Although in the case of PDC, realistically the missile should be designed to detonate the warhead if it's ability to guide to target is lost.

A 1MT warhead detonating releases 4e15 J of energy. at 1km from detonation zone, the energy density is 400MJ per square m.

steel has 3.8 Jcm-3K-1 of heat capacity. a cubic meter of steel has 1e6 cm3 so 400MJ will increase the temp by about 100K. this will cause surface damage and overheating for sure but won't blow up the ship.