Trump has been the President of the United States now for roughly six months. And one thing that we have come to understand about him is that he is NOT reluctant to single out people or institutions that he sees as enemies.
Even when it is not in his best interest to do so, even in instances where party members, advisors, or even family members urge him to show restraint, he insists on calling out people as enemies. He has called out members of Gold Star Families, a sitting senator who was a POW, NATO, the Speaker of the House of his own party, the press, any number of alliances no matter how long standing, and even the Contstitution itself.
It is part of his political DNA. It is fundamental and irreducible to his brand of "leadership": if he thinks anyone or anything is an enemy, he will publicly name them with no hesitation, with no regard for previous custom or any notional propriety.
So the fact that he is somehow reluctant to name American white supremacists as enemies is quite easy to explain: he simply does not agree that they are his enemies.
In order not to see that, you have to not want to see it, or not care.
It is as clear as day.