If we chain two DES encryptions together, the Initial permutation IP and reverse initial permutation, IP^-1, cancel each other out, thereby weakening security.

This is for historic reasons. In fact, encrypting three times would be more secure than encrypting, decrypting, and then encrypting again

Hardware optimization is easier in the case of alternating encryption and decryption, i.e., using EDE instead of EEE.

This allows 3DES to be backwards compatible to 1DES.