I would guess a lot of it has to do with not being prepared (or not wanting to prepare) to handle taxation, benefits, employment regulations, holiday schedules, etc. in unfamiliar or potentially different contexts. You might be able to address some of this if they’re willing to have a corp-corp arrangement, but then you could run into contracting restrictions if it’s a longer term relationship.
There could also be an assumption that they would get international requests from places without great time zone overlap even though that doesn’t apply in your case. I’ve seen job postings where a large number of respondents submitted from India, for example. You could put your time zone on your resume or emphasize it in a cover letter to help address the common US (ahem) skill with geography.
An additional consideration might be travel costs for the times they want you to spend with your colleagues, customers, or professional cohort (e.g., conferences).