In the beginning of The Last Jedi, the Resistance evacuated from their base on D'Qar, just before it was annihilated by the First Order. Then, throughout the movie, they tried to run away from the enemy ships and find a new base for themselves, in order to call for assistance.
Leia Organa: We need to find a new base.
Larma D'Acy: One with enough power to get a distress signal to our allies scattered in the Outer Rim.
In the end, the remaining Resistance survivors landed on the planet Crait and were able to send out a distress call, though there was no answer from anyone.
Kaydel Connix: Our distress signal's been received at multiple points, but no response.
Larma D'Acy: They've heard us, but no one's coming.
Was the signal actually heard by the allies? Why didn't they answer? Were they already controlled by the First Order, like the opening crawl stated:
Having decimated the peaceful Republic, Supreme Leader Snoke now deploys his merciless legions to seize military control of the galaxy.