why didn't the rebels upgrade their fighters between RTOJ and TFA?

If I understand the timeline correctly it's been 30 years since RTOJ and TFA. In terms of technology, even in our own universe that's an eternity. So why is the rebellion, which has a fair amount of support from the New Republic, using fighters that were developed during episode 3 time frame? 19 years before a new hope. So by the time of TFA the rebellion is using a fighter that is about 49 years old. In terms of military fighter craft that's an antique. via /r/StarWars http://ift.tt/2fOz2ry