Sure, "no reason" if literally the only problem you have to solve is deploying docker containers. While I'm not taking anything away from DO -- it's actually not a bad alternative for people who don't want nor need the complexity of enterprise cloud solutions -- it's still a huuuge exaggeration to say "no reason" when Docker containers is just a drop in the ocean (pun intended) of the scale of problems that AWS (attempts to) solve.
Heroku is non-leaky in it's use of AWS. If they switch to something else under the hood you wouldn't know and wont need to change code. The only leaky part is the pricing of someone who is buying wholesale and selling retail.