Howard Wasserman has an interesting post on the causes of structural constitutional amendments. I agree with most of his points. For example, history does indeed suggest that structural constitutional amendments tend to happen in the aftermath of a crisis that is perceived as revealing a flaw in the existing constitutional structure. I would add two other key points:
First, structural constitutional amendments are usually driven by political elites. The rationally ignorant general public rarely knows much about or cares about structural issues. It's not just that the public usually doesn't know much about the details of these questions - that is true of most political issues. It's that they don't care much about them either, failing to see the connection between constitutional structure and effects on their own lives. That makes structural constitutional amendments very different from individual rights amendments, such as the Equal Rights Amendment, which many in the general public do often care about intensely, whether or not they know much about its possible effects.
Second, because of the supermajority requirements of Article V of the Constitution (the support of 2/3 of both houses of Congress and 3/4 of state legislatures), structural amendments can only pass if they have overwhelming bipartisan support among elites. Thus, a structural amendment is unlikely to pass if it is seen as benefiting elites from one of the two major parties at the expense of those in the other.
For example, Wasserman mentions the failure to pass an amendment reforming the electoral college after the debacle of Florida in the 2000 election, when many Democrats expressed support for such an effort. The reason for this failure is clear: in the status quo, the electoral college gives a slight advantage to the Republicans because their are more small Republican-leaning states than Democratic ones. Thus, the Republicans benefit from the electoral college rule that disproportionately weights the votes of small states (each of which get two extra electoral votes for their two senators even if their population would normally give them only one or two electoral votes instead of the current minimum total of three). For this reason, Republican political elites are likely to oppose efforts to pass an amendment abolishing or restructuring the college. Knowing that any such effort is doomed to failure, the Democrats are unlikely to seriously push for it. If the general public cared about the issue, the outcome might be different, because they could potentially force the Republicans to give in, or at least give the Democrats an incentive to push the issue in order to attract public support. But since most voters either don't know or don't care about it, the Dems haven't made a serious effort to press for an anti-electoral college amendment.
By contrast, elites in both major parties had reason to support the Twenty-Second Amendment (limiting the president to two terms) and the Twenty-Fifth Amendment (revamping the rules of presidential succession), the two most recent important structural amendments that passed. As Neal Devins and I discuss in this article, the Twenty-Second Amendment was initially driven by Republican anger at FDR's unprecedented violation of the norm of only serving two terms. But many Democrats could also support the proposal because, going forward, there was no reason to believe that Democratic presidents would be hurt by it more than Republicans. And political elites from both parties could appreciate the potential dangers of allowing presidents to accumulate quasi-dictatorial power by serving unlimited numbers of terms. Republicans especially feared such accumulation by Democratic presidents; Democrats especially feared it from Republican ones. Thus, enough Democrats were willing to swallow the apparent posthumous rebuke to FDR to enable the amendment to pass. The Twenty-Fifth Amendment was even more clearly bipartisan, since neither party had anything to fear from it, and both could appreciate the need to provide more effectively for the death or incapacity of a president in the wake of the trauma of Kennedy's assassination - the event that triggered the amendment's passage.