Nehlen, who's hoping to unseat Paul Ryan in the 2018 mid-term elections, has a long history of overtly expressing his racist views. Twitter suspended him for a week in January over anti-Semitic comments, and he has regularly promoted white supremacist ideology. In private, he used direct message groups to coordinate harassment campaigns. Breitbart supported Nehlen's ultimately unsuccessful run against Ryan in 2016, but distanced itself from him in December 2017.
The candidate hadn't formally commented on the ban as of this writing, but a Facebook post of his tried to portray the suspension as censorship.
This doesn't mean that Twitter has become more proactive or consistent in its enforcement. The ban came when Nehlem's racism was garnering a lot of publicity, and long after it was clear this behavior was par for the course. Still, it shows that the company is willing to take action against notable political figures when they violate its policies... to a degree.