I think you have several problems with a DC show..
While it would be good (potentially) for political exposure.. it would be terrible for exhibitors I think.. People hate DC far more than they hate Atlanta as a general rule.. Its congested, with horrible traffic.. the hotels and restaurants in the areas around its larger convention type venues are difficult to get to, parking is an absolute mess, etc..etc..
The Walter E Washington Convention Center in DC is the only venue I can think of big enough to host an event like DSC.. and its in a particularly horrible area (IMO).. its right downtown.. I think the cross streets are H Street and 11th..
If people wont travel there to buy stuff, exhibitors wont go there to sell stuff.. and then there is nothing for the politicians to attend..
DC is also notorious for protests.. all of the major activists groups watch DC very closely for anything "protestable".. whether its a meeting of world leaders, a summit of economic entities, or some "convention" they might not agree with.. .
Quite literally activist groups that have nothing to do with DSC and its mission would show up in huge numbers.. just because its another opportunity to get some headlines... you'd have 1,000 angry feminists wearing pussy hats, standing next to 1,000 people wearing rainbow t-shirts screaming about LBGTQ rights, standing next to 20 people that really hate rhino hunting... causing problems for everyone..
(I lived in DC and worked for a somewhat controversial organization that got "protested" fairly routinely for a while... it was common to have a dozen different groups out there raising hell, toting signs, etc.. that literally couldn't care less about our organization.. they were just networked with the other protest groups.. so when any one of them had a reason to be upset about something, all the rest would join in and try to cash in on their 15 minutes of fame as well)..
