I suspect US seed sellers have smaller markets vs. the much more established and larger international sellers. They likely pay more for seeds, may have to buy from distributors rather than the seed companies due to their orders being too small, and lack other benefits of being a major player. Plus, US sellers need to stash money to deal with any legal problems, cover costs of imported seed shipments that get confiscated, must deal with more demanding and spoiled US customers (expect rapid service from stoners), have to deal in cash/can't buy or sell on credit, have to self-insure their business (can't buy commercial business insurance), etc., besides lacking established reputations and no US seed company I know of actually having as broad selection (of autoflowering seeds) as some of the European companies have. Plus as legal-operating companies, the US seed sellers have to comply with local, state and federal laws and regulations. And even when US seed companies 'carry' a product line, they usually just offer a small selection of that company's seeds.
So until things are more legalized in the US, I expect the big European seed companies will continue to have better prices, selection, more payment options, etc.