Since these are primarily pitched as user conferences, it's not all that surprising; there's little to no overlap in the user bases, since the products are, by design, more or less mutually exclusive. The group of people for whom this represents a conflict (platform-agnostic nonprofit developers/consultants) is small, but very important to the ecosystems of these products.

Convio v. Salesforce is the especially confounding one, what with Common Ground having recently come out. Looking at the announcement dates, I suspect that was a communications problem, as the dates were announced very close to each other.

BTW: Dev Summit attendees may be interested in the free keynote and expo pass for Dreamforce.