The technology curve in the SFU is such that, every time the base tech level advances into a new era, many of the last generation's designs end up being left behind.
By and large, the SFU follows an adapted version of GURPS' Tech Level chart, with adjustments added in specifically to fit this setting. The "modern" era in the Alpha Octant is listed as Tech Level 12, and covers the ships currently in A Call to Arms: Star Fleet.
The era of faster-than-light travel began for the planet Earth in Y1, with Zefram Cochrane's first warp flight (and the subsequent First Contact with the Vulcans). For the next several decades, Earth ships could travel using fusion-powered non-tactical warp drives only. Those allow a ship to go around nine parsecs a day from one part of space to another, but must slow to sublight speeds in order to engage in ship-to-ship combat. And when they were so engaged, they fought using primitive lasers and atomic missiles. This era is set at Tech Level 9, the non-tactical warp or Q-era. (These older engines live on as impulse drives on modern starships, so a ship which is forced to drop its warp nacelles can try to limp home using non-tactical warp.)
The onset of antimatter-powered tactical warp drive came for the Federation in Y62, when a Terran light cruiser was used as a testbed for the new technology. (That same cruiser hull would eventually evolve into the modern-day Texas-class CL.) The first warp-refitted ships were faster on the strategic map, and were able to fight at (low) warp speeds in combat. They were also able to support the development of new "warp-class" weapons, such as phasers and photon torpedoes. (In SFB, you have to use warp power to arm a photon, be it from the warp engines, or from auxiliary warp reactors fitted to bases or certain types of ships.) This is the Tech Level 10 era; otherwise known as the warp-refitted or W-era. Most of the ships of this period are older TL 9 hulls refitted with the new warp drives, and with their old "sublight" weapons stripped out and replaced with phasers and suchlike.
In Y79, the first ships built from the keel up to incorporate tactical warp drive entered service, starting with the Republic-class early cruiser; the first ship built with the classic saucer-and-nacelle pattern, designed from the outset to serve as part of a unified Star Fleet (as opposed to the separate planetary navies the W-era hulls had first served in). This is Tech Level 11, the early warp or Y-era. In some cases, the older W-era hulls could be upgraded to the new tech level; the Terran WCL got a new set of engines, to turn it into the YCL. However, some other ships were simply incapable of going any farther than they already had. One example is the old warp-refitted Terran frigate, which could not be made to go any further than it had already been. Others may or may not have been upgradable from an engineering perspective, but were economically (or politically) unfeasible in the new era.
Then, with the onset of ships like the Constitution-class heavy cruiser (and the Texas-class light cruiser) in the Y120s, we make the jump to TL12. In the modern era, the earlier ships of the Star Fleet ran into the same problem which had befallen the old Terran frigate in the last jump forward; the likes of the Republic-class YCA simply could not adopt the new and modern warp drives. (There was a different "R"-series class of transitional cruisers, the Republic, Ramilles, and Reshadjie, which did end up serving as "modern" CAs; but this Republic and the earlier Republic are most likely different ships.) Some of the Y-era Star Fleet ships ended up in the various National Guards (the modern-day successors to the old planetary fleets), but others ended up either as floating museums or as fodder for the breakers.
The onset of first-generation X-technology (Tech Level 13) in the early Y180s follows this trend. While the possibilities of X-tech transform the way in which each Alpha Octant empire goes about its business, there are two key limitations which define the ships of this new era. On the one hand, the X1-ships are all upgrades of older hulls, attempting to integrate technologies that none of the original designers had ever envisioned being implemented into these designs. This led to a few discoveries which set X1-ships along a certain design trajectory; you couldn't upgrade a dreadnought to full X-tech, you can't put X-tech into a gunboat, you had to work out how to run X-ships alongside non-X ships, and so on. On the other hand, you need a whole new set of resources and raw elements needed to make X-tech work, and a whole new set of industries in order to process and refine them. This meant that no empire had the means of converting their fleets to X-tech overnight; all the money in the Federation wouldn't help if it couldn't be channeled into a logistical structure capable of spending it.
(In Federation and Empire, you need to raise and spend XTPs, which are distinct from standard EPs, in order to build and maintain your advanced technology units.)
If you want to see what the true application of X-tech will be, you'll have to wait for Tech Level 15, the X2-era, starting in Y205. (Tech Level 14 is ear-marked for some exotic technology types out in the SFU, and is skipped by the Federation.) That is when the first ships built from the keel up to make the most of advanced technology will enter service. However, we will have to wait until the long-awaited SFB Module X2 is committed to print before those details come to light; and no-one can say when ADB will manage to get that one up and running...
So, to put a long story short, each jump from one technology level to another has its issues; and in the case of the Gettsyburg and Kirov/Bismarck/New Jersey, the ins and outs of first-generation X-tech are such that one class can go on to grander things as the Vincennes-class CX, while the other finds itself on the wrong side of the TL 12-to-13 tech divide.