Earlier quoted context omitted.
Citizenship is the primary determinant of ESTA, because ESTA is the feeder of the Visa Waiver Program which in turn is based entirely on citizenship. Everything else in ESTA is largely just duplicating requirements that would be imposed without ESTA (such as remaining validity of passport, whether you have a return ticket, purpose of visit, etc.).
The Visa Waiver Program was in place before ESTA was introduced. Before that, the airline took a look at your passport and then a list of allowed nationalities. Having the traveller fill out an online form does not change this requirement.
ESTA's purpose is to determine eligibility for VWP. VWP is based on citizenship of a participating country.
All other things asked for and evaluated by ESTA are things which any traveler, entering via any legal means, can and usually will also be evaluated on. For example, ESTA will usually verify that you have a passport which is valid for at least six months beyond your scheduled date of departure from the United States. This is not a unique requirement that ESTA, only ESTA, and nothing else anywhere other than ESTA imposes. A minimum amount of remaining passport validity is an incredibly common requirements, not just of the United States but of practically every country in the world.