If we are talking money incentives, a community that are discussing stock investments seems more interesting for advertisement than politics. If a person are investing in stock options, it almost guarantied that they have spare money.

Everything surrounding TD subreddit was getting out of hand so they had to act to look like the good guys who care about stuff.

They know they have more to lose by pissing off the community.

The Donald subreddit was big, but the writing was on the wall that Trump was about to lose, and /r/politics + /r/SandersforPresident + /r/AOC have always been bigger and more popular. It was a very easy decision for reddit.

The user count on r/thedonald was frozen as to not influence popularity, you are likely wrong about their relative size.