Dill pickles are crunchy because the cucumber utilized in the pickling process started out fresh, and the enzymes present in the vegetable did not negatively interact with the fermentation process.
Making dill pickles out of English cucumbers can make the product less crunchy because of an enzyme found in this type of cucumber. This enzyme is found in other types of cucumbers, but because it is highly concentrated in English cucumbers, this type should be avoided for pickling.
To increase the crunch of a pickle, trim the cucumber's blossom end prior to fermentation. Adding grape leaves is another way to stop the negative interaction between the enzyme and the fermentation process.