It is mainly so because of lack of education and emotional manipulation into mixing up human biology and human personhood. Heartbeat doesn’t make a person. It’s not even a requirement to be a person (people with continuous-flow artificial hearts lack it). The first specks of personhood appear a lot later in the process.

A normal sane reaction is “yeah, there’s already a working cardiovascular system, better stop it before the organism develops even further.”

This said, it’s an obviously pointless and harmful delay. There is no reason to check a heartbeat of a fetus if a mother decides she doesn’t want to carry it.