Also keep in mind that many of these planets were part of the RWR. When the Republic collapsed, the Lyrans snatched up all of the nearby, profitable worlds, and the rest were simply left alone. THe Steiners had far more pressing concerns with the looming 1st SW to both maintaining a tenuous control on financially worthless planets on their peripheral border. As such, many of these planets were likely assumed to have either died or simply not have retained enough contact with the IS to warrant even being considered as to whether or not they were still alive.
Also, keep in mind that until the Explorer Corps, Periphery exploration,e specially into such unpopulated sections as the Rift, were last on most state's agenda. And even then, aside from exploring the immediate vicinity that later becomes the Rim Collection, the Explorer Corps' mission in Lyran-ward Periphery space was cut rather abruptly short by Katherine Steiner-Davion when she came into power. And even then, it appears to have received the least of C*'s attentions.