I was just wondering why they did not tell me to go around, when it was clear, even from then, that I was not going to make it.
Perhaps because they though you could make it? Or because they thought, just like you did, that the aircraft in front would take the middle exit?
It is sometimes surprising what experienced pilots can do in a familiar aircraft and on a familiar airfield, and ATC knows that. So they sometimes push the envelope a bit when they are working an aircraft based locally, expecting that an experienced pilot will work with them, an an inexperienced pilot will simply say no.
So in your case, executing the go-around at 300 feet was the right thing to do. But a more experienced pilot, familiar with the field, ATC and the aircraft, might have continued the approach and squeezed in behind anyway.
In situations like that, it's a bit of a game between the pilots and ATC. If you know the rules and have the experience, you can play the game. If you lack the experience, don't play and stick to what you know. You can always go around. It's the landings that are the hardest.