It's a lot more complex than that. In the months after Endor, as the new government was trying to get set up, Mon Mothma was urging de-materializing, so people wouldn't feel/fear the New Republic would just swoop in and govern through military muscle the way the Empire had. By the time of the Battle of Jakku, a year after Endor, Coruscant was a quagmire with the ISB trying to maintain control over a populace in open revolt, with an almost powerless Mas Amedda pretty much a prisoner in his suite. As far as the actual Empire, as a viable military entity, it was in the hands of someone off-planet, who had been mustering the remnants of the Imperial fleet and their sole remaining Command Ship (maybe -- the fate of the
Eclipse is murky... it is hinted that it preceded Sloane and Hux to what would ultimately be First Order territory) at Jakku to get the last of what was needed to kick-start the First Order. That last Super Star Destroyer,
Ravager, went down, along with several other capital ships of both sides. That was the death knell of the Galactic Empire and the New Republic could get on with cleanup and consolidating legitimacy.
The next quarter-century is still a blank spot, but when we check back in about five years before TFA, the New Republic Senate has polarized into those favoring collaborative governance, where individual planets and systems have equal say in things (... which is resulting in a government that is about as paralyzed as the Old Republic Senate had become) and
no military, and those favoring a strong central government with a single leader and waxing rhapsodic about the good old days of the Empire when things actually got
done, with a
strong military. At the end of all the drama, the Centerists, who had tacitly been helping bankroll the First Order (yet to publicly reveal themselves), basically rage-quit the New Republic, leaving them to their touchy-feely legislative logjam.
Leia had discovered the existence (if not the name) of the First Order lurking out there and getting ready for...
something -- probably nothing good. Since her parentage had just become public (thanks to the Centerists), her credibility was practically nonexistent. So she left in a huff, taking those who believed her with her, and set up the Resistance. Over the next couple years, they were able to scrounge some surplus ships and get some backing from a few politicians and worlds who knew her from Rebellion days and knew she wouldn't do all this without a damn good reason.
When we get to TFA, the government has basically been "dead, but doesn't know it yet" for close to a decade. If the Starkiller hadn't taken out Hosnian Prime, the New Republic Senate probably would have disintegrated within a few more years on its own. But the Starkiller
did wipe out Hosnian Prime and its moons, one of which was the main base of the New Republic Navy, where most of their assets were, since everyone had been dithering about what to do with them. So the galactic government and military got 90% taken out in that one shot. We have yet to see the fallout from this, as no time has passed yet. But given all this, it totally makes sense to me that the
Raddus is the only capital ship available right after. There may be others, in transit, or on-station in some system, but they won't have had time to react yet.
[ETA: Oh, and regarding the Rey's parentage thing. I still like the idea that back when Obi-Wan was an apprentice and was protecting Satine, he knocked her up and never found out. She didn't want to be the reason for him leaving the Jedi Order, and was young enough still that the baby was raised by their parents as her younger sister, Bo-Katan. There's a lot of precedence for such things here in RealityLand[emoji769]. Bo-Katan never found out about her true parentage, and later had a child... Who later had a child eleven years after the Battle of Endor... Who for some reason got left with Unkar Plutt on Jakku when she was 8.
Pretty sure it'll not be that way, but I enjoy speculating.
]
--Jonah