There are different reasons different guitarists have become iconic. Not one reason for all of them.
People here seem to be arguing that Alex is a great guitarist. He is. But that wasn't the question, the question was why he wasn't more highly regarded, specifically by non-huge Rush fans.
That question has been answered multiple times, and the responses....again...focus on why he's a great guitarist. Two different topics.
He has MAYBE one iconic riff, and how iconic it is is very debatable. He has no iconic solos. His playing more blends in than stands out. Etc. etc.
For big Rush fans, he has plenty of iconic riffs, iconic solos, and signature style playing. But that wasn't the question. He's already highly regarded among huge Rush fans.
If I ask random rock fans (not big Rush ones) to sing a single Rush riff, they might...MIGHT be able to remember the opening to Limelight, but that'd be it.