It wasn't even a real goal of his. He thought the saying was "hokey".

But they had this expression ‘drain the swamp.’ And I hated it, I thought it was so hokey. I said, ‘that is the hokiest, give me a break, I am embarrassed to say it.’ And I was in Florida where 25,000 people were going wild, and I said, ‘and we will drain the swamp’ — the place went crazy. I couldn’t believe it. And then the next speech I said it again and they went even crazier. ‘We will drain the swamp… we will drain the swamp,’ and every time I said it I got the biggest applause. And after four or give times I said, boy what a great expression, I love saying it, it’s amazing.”

https://thinkprogress.org/trump-drain-the-swamp-cambridge-analytica-steve-bannon-nrcc-09152b7a90b9/