Ultimate Spider-Man doesn't have to be as iconic as 616. 616 is a joke nowadays and a lot of it's character development and iconography has been largely taken for granted or sullied beyond parody or recognition. Ultimate Spider-Man remained the most consistent title of the entire line even when it too became a farce, it's two Spider-Men revolutionised the modern spin on where to take the characters, Miles became a break-out star in an industry that normally doesn't reward legacy characters with that kind of success, and at least it had an ending. People can flock to it, get something out of it, put it down after a while and come out with a satisfied experience, whether they're there for Peter's exploits or Miles'. It is a quicker, pacier and altogether (and at least to a point) less complex entry point into the franchise for many fresher minds.