It's the same reason you don't always want to play in Middle Earth during the War of the Ring, or why you don't always want to play in Star Wars between the destruction of Alderaan and the Battle of Yavin, or why you don't play Star Trek actually on board the Enterprise. You want the setting, but you don't want to retread old ground or use predictable, known characters (that already have their own arcs and aren't even yours!). You want to preserve the setting without being obligated to obey the plotline.
Everybody knows the story behind the Pentad Prism and it's not interesting anymore (if it ever was). Rajaat and Borys are not the attraction of the setting. The easiest way to communicate that to the players is to shift time. That also means you have an opportunity to make changes or corrections to possibly make the setting more appealing to modern players, to give the setting more active content, etc.