There are failed colonies, and then there are radioactive hulks orbiting a star. >:D See how many planets the Capellans lose (not to other States) between 2822 and 3025.
Keep in mind that except for a few limited conflicts, the Periphery mostly stayed out of the Succession Wars. They didn't lose their planets (directly) to conflicts, they lost them due to economic and technological difficulties (There's one planet that was lost/damaged due to a bioweapon attack by the Taurians). The only other chance for them to have been lost to warfare is during the Periphery Uprising in the 2760's, and the Star League's response. I have no doubt that the fighting there was incredibly brutal, and utilized nuclear weapons, but I doubt it was on the Planetary killing level (though again, damage could lead to technological loss, which leads to colony loss).
Is there much detail that describes if these planets broke away or if they simply died out as failed colonies? You'd think if they died out, the Magistracy would be interested in restarting them instead of messing with all the political wrangling with the Taurians over the Fronc region (and that ultimately backfired).
There isn't much data on why those colonies disappeared. As
Nerroth said, there's some data in the most recent ISP book, but only on 2 or 3 planets (including one that wasn't on the 2750 Magistracy Map). Like the loss of most of the planets in the BT universe during the Succession Wars, there just isn't alot of specific data.
As to why they went with new colonies instead of restarting older ones, well, if you look at the maps, they did a little of both. The Magistracy rediscovered/reconnected/repopulated some colonies, such as Vixen and Kossandra's Memory, while they also went into the Fronc Reaches with the Taurians. Politics explains most of the Fronc Reaches. It was supposed to be a joint Periphery project between the Magistracy and Taurians, to reinforce trade and political ties. The Magistracy isn't able to stand isolated, and this was a good way to get room to expand, while having someone else help pay for it, as well as linking to two nations together.
As to why they didn't go for some of the other colonies that the previously had, it could be that they went for the ones that are easier to connect with/to rebuild. Its possible that they ones they haven't rebuilt require technology that's still too expensive, or still not available to the Magistracy to become viable again.
Though an even better question would be what sort of augmentations should there be if you wanted to play as a Catgirl in Time of War.
I was working on an Ebon Magistrate Catgirl a while back for fun, I might have to continue, though its not technically by the rules since I think you have to buy augmentations with XP during Chargen..
