Why? The New Republic is barely in TFA as is, and has been refusing to support the Resistance due to their mistrust of Leia’s lineage.
The NR surving into TLJ and RoS needs no explanation, as those films continue the thread of allies not showing up to help… until they do. With the NR destruction removed the viewer can connect the dots and just assume that they’re continuing to refuse to help, until in RoS the First Order actually has a means of threatening the whole Galaxy.