Here's my take. Vampires, even though immortal, need blood to keep their "look" without aging. Without blood, they will age and become decimated things. I didn't see the 2004 pilot, but remember seeing a photo of Barnabas in the coffin when Willie opened it and he was a shriviled, dried-up corpse. From just my observation of that photo, I took it as to explaining my hypothesis. Barnabas was still "alive," but had taken on the looks of someone that was buried for 200 years. With enough blood, he resumed the way he appeared at the time of his death and would retain it with a fresh supply. I don't know if that's what they were saying in the 2004 pilot since I've never seen it, but that's just my take.
Interview with a Vampire hinted at that. One always looked the way he/she did when he/she died at the hands of a vampire and returned as one. If one even so much as cut his/her hair, it quickly restored, for example. Good thing Barnabas didn't die with a hair-do that looked like a powdered wig. Imagine him trying to explain that in 1967. It wouldn't even pass in Haight-Ashbury.
Gerard