Basically, because people thought it was safe. It didn't seem significant to use the same prime since the key exchange process involves both Alice and Bob gener

Basically, because people thought it was safe. It didn't seem significant to use the same prime since the key exchange process involves both Alice and Bob generating a secret. Solving the discrete logarithm of g^a mod p was still thought to be intractable given a large enough (1024bits) p. As to the technical reason why the DH parameters were reused - speed and efficiency. A DH key exchange would take longer if you had to generate those parameters each time.