I don't think any Democratic politicians literally told BLM to "go forth and riot". That's the difference. If someone is sympathetic to a movement, it's hard to say they had a hand in events. If someone literally says "hey everyone, let's do this thing!" then it's very obvious.

You could also argue that Trump's previous tweets, the wink wink nod nod tweets to white supremacists, also fall under a similar category as the BLM tweet. Sympathy and possibly support but not actual incitement.

This time was different. Trump was literally inciting sedition. "Go to the capitol and stop the steal" was the message.