Two Reasons For This

There are two reasons why legislators think that their constituents are more conservative than they actually are.

The article correctly identifies the proximity of wealthy donors to legislators.  If you are running for office, you need money and the money at the state level is most likely to come from the sort of entrepreneurial characters that make up the Main Street Republicans.

But I think the other reason has to do with the vocal nature of conservatives.  They write their local newspapers.  They call into talk shows.  They astro-turf their representatives offices.  Check out the comment thread of a local newspaper or even Yahoo!

I have no idea why conservatives tend to be disproportionately vocal, but it's likely tied to their increased presence in the midterm elections.  Whether it is because they feel under siege or feel their positions more strongly than others, they are certainly more visible.