Of course, the real reason why all the other Riders are conveniently dead or shortly to die of Wise Old Mentoritis (or evil and absent for 99% of the plot) is to make Eragon extra, EXTRA special and important. Because it's not enough to be an immortal super warrior with all powerful magic, an unbreakable one of a kind custom made magic sword and a dragon to ride - you also have to be the only guy around who gets all that, because author forbid you have to share the spotlight with anyone else.