The New Republic needs to be set up as fragile in order to justify why a militia such as the Resistance is necessary in the first place. They’ve put all their eggs in one basket basically, which is to protect the Hosnian system.

We also don’t want Senators to outright support the Resistance, as this would clash with Leia’s scene where she considers that most of them think she’s crazy and won’t listen to her. It’s better that they’ve only commissioned and helped with the mission to find Luke Skywalker, rather than supporting an all-out war with the First Order.