I really think it is because people were aligned to the political causes of those riots and were willing to use violence to achieve their political goals (which happens to be the dictionary definition of 'terrorism'). Even those who were not committing violence themselves, like politicians, were willing to excuse others engaged in violence. Here are four quotes from politicians inciting rioting in a much more explicit way than Trump - I say more explicit particularly because by the time they said these things, there was already widespread rioting: https://twitter.com/BernardKerik/status/1347652272852004877 (Ayanna Pressley, Kamala Harris, Maxine Waters, Nancy Pelosi)

The violence was also not just confined to May. In Seattle, there's a group called EDM (Every Day March) that conducts illegal riots blocking highways every single day (literally on over 200 days so far since George Floyd). They organize themselves on social media, particularly on Instagram where they have two accounts and document/evangelize their own crimes (https://www.instagram.com/morningmarchseattle and https://www.instagram.com/morningmarchseattle). Note that Facebook has not banned them, GoFundMe allows them to fundraise on their platform, and local politicians support their actions and false claims (example https://twitter.com/cmkshama/status/1288511663948230657).

Another example is Rose City Antifa in Portland (https://en.wikipedia.org/wiki/Rose_City_Antifa), which is the oldest and most active antifa group in the US, and has been engaged in riots throughout 2020. They have done things like beat a journalist who was documenting their crimes (https://thepostmillennial.com/judge-dismisses-anti-slapp-mot...), set fires at a courthouse (https://www.nytimes.com/2020/07/21/us/portland-protests-dama...), vandalized federal buildings (https://www.dailymail.co.uk/news/article-8670207/Portland-pr...), and repeatedly assaulted police officers. This disgusting terrorist group still has a presence on Facebook (https://www.facebook.com/sometimesantisocialalwaysantifascis...) and Twitter (https://twitter.com/RoseCityAntifa) even though they are explicitly and consistently violent.

As an aside, isn't it interesting that people are still gaslighting everyone by claiming that antifa is "just an idea"? When's the last time an "idea" showed up to a court and filed a fraudulent anti-SLAPP motion after viciously beating a journalist?