"Why do you think Twitter hasn't enforced their own Terms of Service rules when it comes to Donald Trump?"
Well: Trump equals attention, attention equals visitors, visitors equal ad impressions, ad impressions equal money.
In case someone didn't notice:
Attention (not regard or esteem) has become a de facto currency in the last years.
How do you think people like Limbaugh, Beck or Trump can make a living? From honest work? Please don't make me laugh. They use their hate and bully-ness to create attention which they monetize.
You find the mechanisms at work in Gamergate or Puppygate or whatever the attention whores come up with