Why do Panasonic model sizes differ per region? Why fewer sizes for high end?

Yeah, it has to do with several factors; those that come to mind, foremost, are home sizes and economic considerations, which vary by region, of course.

For instance, in Japan, homes, which are typically apartments, are very small by U.S./Can. standards, so they sit closer to the screen, thus they don't need or even want, as big a screen.

Furthermore, sitting close to a smaller screen, (what we in U.S./Can. might consider 'medium-sized'), they can appreciate, and want to benefit from, the increased picture quality of a VT series, for instance.

Whereas, in U.S./Can., indeed, people who buy such medium-sized screens, are rarely anymore video enthusiasts, so it wouldn't be cost-effective to make VT series in anything 50" or less.

From what I've read, the 50" V series weren't selling that well in U.S./Can. during the past two(?) model years, anyway, as there were too many leftover TVs.

North American sales history over the past few years dictated the sizes and models for 2011. There is not enough demand for 42"/46"/50" higher end or flagship models. Witness the large number of surplus 2010 42" and 46" GT25s and 50" VT20s still being sold 6 months after they were discontinued.

Indeed.

However, though production may have stopped on those TVs 6 mos. ago (early Dec.), there was/were no viable replacement(s) for them until the ST30, which was only released ~3.33 mos. ago; still too much time with surplus, though.

50" GT30s are available in the US.

Yes! And I was fortunate to get one of the GT25's from Costco!