Its just the business cycle - I really don't know why, it's just one of those "we have always done it this way" sort of things. Part of it I think is that people take time off (customers and marketing managers both) and travel, part is that it looks low because many marketers get a little panic-y and so decide to do more marketing and market research near the end of the year when there is both holiday shopping and the end of the year in terms of figures for bonus calculations etc.

Typically, you would be of interest to more marketers in a large city than a small town, and a populous state rather than a small one. (Just because you are likely in the market for more goods and services in those locations than in a smaller area with limited supply / competition). That said, there will be surveys that look for all, however quotas for a national survey will fill very quickly for minor demographics so even if you get sent a survey, you are less likely to get through than if you are in a large demographic.

In US phone surveying, the oldest age groups and women are usually the first (non-geographic) quotas to fill, and its usually very hard to get non-anglo groups. I would imagine that online surveys are inverted - age at the younger end, more men than women, although probably still heavily anglo.

Curiously, incentivized surveys are typically thought to be a poorer indicator of opinion than those with no incentive due to the higher chance of false responses. However, they are quick, and no doubt cost effective.