What is it exactly though? Let's face the inconvenient truth; in the end, it's all about popularity. Again, a certain member in the past made a good point by posting the following in another thread (something for us to think about):
"All of the factors contributing to greatness are interrelated and dependent on each other. For example, one factor mentioned above is the tradition of received wisdom: belief in A's greatness has been passed down from generation to generation, reinforced by music textbooks and concert performances and internet forums, while belief in B's greatness has not. Another factor mentioned above is the test of time: A seems greater than B because the former's music has survived till today while the latter's has not. But these two factors are mutually reinforcing: if music textbooks have chapters on A but not B, then of course the former is going to have a leg up on the latter when it comes to the test of time. Conversely, if A's music is still performed today while B's is not, then of course music textbooks are going to have chapters on the former but not the latter. Likewise, another factor that has been mentioned is influence: A has demonstrably had a lasting influence on later composers, even today, while B has not. This is also inherently connected to the above factors: since A appears in textbooks and is more widely performed than B, then of course he is going to have a greater influence on later composers than B will.
In other words, the concept of greatness is a complex and circular system. By this point in time it's also a self-sustaining one, precisely because of the circularity. After all, this system is basically what we call a canon, and it is the very purpose of a canon to be self-perpetuating. As I wrote about in another thread some years ago, it is difficult to imagine any canonical composer being removed from the cycle and losing their canonical status, and it's difficult to imagine any non-canonical composer being inserted into the cycle and acquiring canonical status. I don't think the canon was always closed, and I don't want to think it is now, but if I'm being honest with myself then I have to think realistically that it is."