Why is DES secure against differential cryptanalysis
It's mainly due to the use of multiple rounds and the presence of the various permutation and expansion mappings that bring the overall maximal differential probability to a very low level for key recovery.

This then leads to unrealistic requirements for chosen plaintext/ciphertext pairs encrypted under the same key, in ECB mode.

And the presence of different sboxes complicates things a bit further. Under current technology, or even for the last 20 years, a simple brute force key search with a small amount of plaintext ciphertext pairs is much simpler and easier.