I noticed this store had a mixture of mostly what looks like 4' Philip fixtures and the 8' lights were all Sylvania's. The 8' lights are scattered in random places around the place. I think maybe they were used where more distribution may have been needed? A couple 8' tubes were missing too.
They were probably all 8ft fluorescents at one point and retrofitted to 4ft T8 tandems as they failed. 4ft tubes are a lot easier to handle and when one tube goes out it's not as drastic of a reduction in light levels as an 8ft'er going out. I think those could be two of the reasons 8ft T8s are uncommon. Could be a "one size fits all" agenda though. Personally I think the single pin sockets are more appropriate for T8s because of the high OCV ballasts. Bi-pin sockets are intended for preheat or rapid start circuits, which are under 240V. Electronic T8 ballasts are typically 600V. If the lamp is not installed correctly the socket will arc and melt. With rapid start or preheat that's impossible since both pins are used to properly complete the circuit. Instant start only needs one of the two pins, hence the single pin for slimline lamps. You can't install them wrong either with single pin plunger sockets.
F32T8s are bi-pin though because they're intended to be used for retrofitting F40T12 fixtures. Makes upgrading much easier. Single pin 4ft T8s exist (F48T8 Slimline) but I have never ever seen one used. They're no different from an F32T8 as far as I know aside from having a single pin base instead of the bi-pin base.
I've noticed that most places here with 8' T8s had lights that were retrofitted, all the stores I've seen with new lighting use tandem T8 strips instead. Yeah I prefer 8' T8s over the tandem retrofit kits, retrofitting to 8' T8s seems to be less work anyway so I don't know why those retrofit kits are so popular.
I could be wrong but I think one of the reasons why T8s are bi pin is that they were originally designed for magnetic rapid start and it was only later that they made electronic instant start ballasts for them so they kept them as bi pin to remain compatible with the earlier RS T8 installs. Personally I'd make Instant start only T8s with internally shunted pins and beefier electrodes for instant start and universal T8s with conventional electrodes and non shunted pins but then you'll get the guys that put a shunted T8 on a rapid start ballast and blow the ballast.
Speaking of rare T8s, Sylvania makes (or made) a 4' T8 with RDC ends for retrofitting F48T12HO. They were intended to run on a HBF F32T8 ballast to match the lumens of a T12 HO.
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F32T8s are bi-pin though because they're intended to be used for retrofitting F40T12 fixtures. Makes upgrading much easier. Single pin 4ft T8s exist (F48T8 Slimline) but I have never ever seen one used. They're no different from an F32T8 as far as I know aside from having a single pin base instead of the bi-pin base.
I could be wrong but I think one of the reasons why T8s are bi pin is that they were originally designed for magnetic rapid start and it was only later that they made electronic instant start ballasts for them so they kept them as bi pin to remain compatible with the earlier RS T8 installs. Personally I'd make Instant start only T8s with internally shunted pins and beefier electrodes for instant start and universal T8s with conventional electrodes and non shunted pins but then you'll get the guys that put a shunted T8 on a rapid start ballast and blow the ballast.
Speaking of rare T8s, Sylvania makes (or made) a 4' T8 with RDC ends for retrofitting F48T12HO. They were intended to run on a HBF F32T8 ballast to match the lumens of a T12 HO.