Seems odd to me. The Rig I am sure had alot of supplies as well. It had labs and they could build advanced pa. Their plan to use the Jet Stream to send Modded FEV around the world did not come about till around 2240-2241. So what was the Enclave doing for 164 years? I don't see their plan in FO2 being the reason for siding with the Rig over the Rock.
That's the point, why would they need to leave later?
They could have set up at Raven Rock. Set up away to watch the Vaults. Used the FEV from Vault 87 and used the people of Vault 101. If they really needed to they could have sent people to Mariposa and Vault 13. They could have gone anywhere in the USA they wanted, based on how FO3 makes travel look so easy. I doubt the Rig was the only place they could send FEV into the Jetsteam. Very least they could have abandoned the Rig and went to the Rock at any point up till FO2. They had 164 years to do so. They are crazy about taking over DC in FO3. For people that are "the government" they sure did not care about DC for over 164 years. They could have used both bases. They had contact with Eden seems they knew DC was in ok shape and the Rock was still there. Enclave being "the Government" would have known about optimus prime at the pentagon as well. So the Oil Rig is great but DC is the Capital. It has way more Vertibirds and other equipment. It has Optimus Prime and a Land Crawler Base as well as control of a Space Weapon and yet they ignored it for over 164 years till the rig blew up? With the Rig and Raven Rock the Enclave could have taken back America a long time ago..
They started the plans for FEV in 2230's when they first ventured back onto American soil to see the choas caused by roving gangs of ex-Unity Super-Mutants, along with the would-be warlords battling for the wastes. Fallout 3 provided no details on the travel between the two, personally I don't really see the need in splitting the nation in two, the Enclave relied on propaganda and such to keep up moral and prevent desertion.
They certainly were [censored] crazy I'll give you that, they wanted to use the Purifier to kill off all of the mutants so that they could begin rebuilding, that rebuilding could have taken place anyway though, the fact that it is the Capital means nothing to the Enclave in the short term. I'm still sure that the Oil Rig had satilite communication, they could easily have controlled Bradley-Hercules from the Rig.
Any how it's a moot point really, when the Enclave was first created there was no Raven Rock, no Optimus Prime and Bradley-Hercules, it's just another Fallout 3 induced mystery, personally I really don't see the need in having both, the only emerged for the first time in the 2230's and thought about solving [censored] from the Rig.