The CR-70 was introduced to the galaxy as a Corellian corvette-class capital ship and a benchmark of luxury. Unlike later ships in the CR series, the CR-70 was primarily created to be a diplomatic and transport vessel only and was designed to meet the needs of the upper class on many levels. Designers believed that in this role the ship would never have any reason to travel outside the Core, where at the time, piracy was almost nonexistent.
Instead of installing weapons systems, the CR-70's designers made full use of its power core in other areas. Extra available power allowed for a special acceleration compensator to be added to the engines which increased the sub-light speed of the ship and reduced the distracting vibration of the engines. The results were engines that made the Corvette as fast as fighters a tenth its size while maintaining a smoother ride than other ships in its class; its speed often gives it the ability to outrun enemy ships.
The sub-light engine was not the only area where the additional power is diverted on the CR-70. It also boasts an impressive sensor array, a military grade shield generator and a slightly larger hyperdrive unit, which gave it a small boost over its sister ships.
Another added benefit of not having weapons installed on the CR-70 is that the space normally taken up with a weapons system was given over to a larger passenger capacity and several more escape pods. The extra capacity enables high society to transport their entourage and a multitude of servants, while still maintaining enough cargo room to support them on extended trips or diplomatic missions.
When production of CR-70s was eventually halted to make way for the CR-90, a retrofit was made readily available to owners to upgrade their CR-70. With the advent of the Galactic Civil War, and the increase of piracy due to the lack of a stable government to stop those who'd prey on others, most owners felt that the ship presented a too large target to be left in an unmodified state. For those who could afford it, the upgrade was done and it is for this reason that CR-70's are rarely seen in the galaxy.
The CR-70, while rare, can still be found and is often considered a collector's item for those who can afford it. This being the case, it is viewed as an aging design that has found itself a niche in our galaxy.