The short answer is the "14-day" rule, which doesn't allow development of the embryo beyond 14 days. The article gives specifics under the heading "Ethical and legal compliance"

In the US it's legal-sorta but the NIH can't fund it and the FDA is not allowed to approve treatments based on it. So someone could do it in a research setting but there's not a pathway to market in the US (in practice people will do the first ones in a friendly legal climate like Peru).