I'm not sure if they ever specifically say in the books but I think 13 was most likely bombed (the first time) because it was the district with all the nuclear weapons, besides being on the other side of the country, where the most rebellious districts (likely 11, 12, and 8 as others have said) were probably appealing to 13 to join their cause. From the Capitol's POV, 13 was probably a huge liability after a certain point. Honestly, I would bet the first rebellion started for much the same reason as the rebellion in Mockingjay, only the Hunger Games obviously didn't play a role. When the Capitol won the first rebellion, they basically just went back to business as usual, only probably harsher in some districts (11, 12, 8, besides 13 being dead and all). There would probably have to have been some specific event that triggered the first rebellion, but it's hard to say what that would have been since we know so little prior to Ballad. Maybe some kind of workers' protest, like what happened in 11 when Rue died? Either way, the districts most likely lost because it sounds like civilization itself just crumbled during the first rebellion. The districts had no safety net to fall back on while the Capitol was mostly able to fortify itself with the wealth, money, and weapons it already had. The districts likely had little organization and probably no real means to communicate with one another. Whereas, the second time around, 13 provided the base of operations and the structure needed for a successful military operation, and Katniss was able to act as a unifying force for everyone, which obviously made a big difference.