Dell was the number one comic publisher at the time of the code and they were very family friendly, so they saw no reason to join a code created by lesser competitors. Also retailers had no problem ordering Dells regardless of whether they had a code or not, the books would sell.
Up until mid-1962 Dell had Western Publishing put together their comics, then the two companies had a falling out & Western created Gold Key, while Dell pretty much had to create a new comic book company, but using the same name. Neither Dell or Gold Key were subject to the Comics Code.