I suspect it probably has more to do with business technology rights than than real technology but here are a few logical reasons. Plasmas used to be a lot dimmer than LCDs. I remember when LCDs looked brighter and crisper and stood out a lot more on the sales floor. By comparison plasmas looked very boring. These days it's difficult to tell the two apart.
To be honest, I didn't even know plasmas were still around until you brought it up. I thought it was a technology that had vanished years ago along with rear projection stuff like DLP. The last few times I saw plasmas in electronics stores they were always in some back corner on clearance.
My guess is that it was a new plasma that hadn't been broken in yet and didn't have the anti-image retention features enabled. Pixel shifting and such should prevent burn-in, and it's transient in modern plasmas. Plasma phosphors tend to be hotter when you first buy the TV; they harden and cool down a bit after maybe a couple hundred hours of use, which dramatically reduces the chance of image retention. The newest plasmas shouldn't require much of a break-in period, if any at all.