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.