Back when the 57's and 67's first came out, I happened to be in Seattle for a period of time. I met somebody who knew their way around Boeing, and he gave me the direct dial number of one of the desgners of these planes. I called him and got straight through. He was rather astounded to be contacted directly by a Frequent Flyer (this was before SkyMiles), but was nice enough to talk to me for a few minutes. I explained how inconvenient it was to be without these lights, paricularly this time of the year. His explanation was pretty much what has already been posted, namely that on these planes, the APU (alternate power unit) does not have enough power to start an engine and maintain cabin lighting at the same time. This had not been an issue on other Boeing planes up until then, apparently because the APU's on the 727 etc. were relatively larger in comparison to engine size.