The reason is quite simple really: shade. In the top photo I am standing on the south side of the trellis. Both years we used this design, the tomato plants on this side did quite well, the ones on the other did somewhat crappy.
The north side tomatoes were destined to be smaller than the other side just because it wasn't getting that true southern exposure, but the unforeseen problem actually turned out to be moisture related. By 11:00 in the morning, the sun would be blazing everything quite effectively. However, one morning (probably around 9:00) when I walked past the north side of the trellis, I noticed that the morning dew had not yet been evaporated; about a week later the same dewy leaves were turning brownish yellow and they eventually shriveled up. At the end of the season the north side tomatoes looked like trees from a Dr. Seuss book, tall scraggly trunks with bushy floppy tops. No good.
We've tried a lot of trellising techniques over the years and it looks like we'll be trying another this year. I believe that if we had the space we would stick with the last trellis from our Downingtown garden.
It had everything: plenty of air flow, plenty of sun, and plenty of support. The only problem is that it took up more room than we can sacrifice in our tiny backyard. Maybe we can develop a city version. Hmm.