Because they sell more to us than we sell to them, they need a deal, and they have always thought there was a distinct possibility the infighting in our parliament would mean brexit would never happen at all which is their preferred outcome.
Johnson has realised to get the EU to climb down he must put us on a course of no deal, for the first time in 3 years the EU can see this is now the outcome if they don't come to the table for the first time and give concessions.
The EU have disrespected us from day one and made it as hard as it could to get a deal out of them we'd be happy with, the EU does not want us to leave, they want control over us, they can see the possibility of us making our own way and taking advantage and prospering from being out of their jurisdiction, showing the way to other countries to follow us out the door.
Johnson is just showing what an absolute feeble PM May was.