I'd love to hear a more plausible rival hypothesis. Anyway, there's plenty of evidence. Well, one such hypothesis is that the Cubs' superior drawing power is due to a variety of factors, not all of which are tangible. In short, the Cubs just have a more appealing brand than someone like the Mets. Part of that is Wrigley field, but I certainly don't think it's reasonable to ascribe all of the extra pull as being due to Wrigley alone. This should be something we can test at least a little bit, although I don't know who might compile this kind of data. Do concerts at Wrigley draw more? Has another team over played "home" games at Wrigley against someone other than the Cubs? How were those numbers compared to their usual? You claim there's plenty of evidence, and I'm generally curious to see what evidence you're thinking of that supports Wrigley Field as the sole factor. The Cubs' drawing ability around the country is based on something pretty tangible; the proliferation of WGN around the country for decades. You couple that with the team coming from a big market that has a lot of expats and retirees to other states and you have most of your answer. I actually think WGN is much better answer than Wrigley, but that may be my own bias since it's mostly how my fandom was stoked.