It was cracked because it depended on a super-increasing sequence, the knapsack problem is still NP-Hard. However, finding a given sequence of primes that sum to the number is not really the knapsack problem.
If I recall correctly, the private key was super increasing. The knapsack encryption algorithm attempted to reduce the super-increasing sequence to a regular sequence. What was broken was that the reduced sequence, while no longer super-increasing, still turned out to be a special case which was easy to solve.
