According to one American poster on the thread I started, the problem is States' autonomy versus federal law. I imagine it would be hard to get a change to the constitution through both houses (even if Trump and the Republican majorities in the two houses were the sort of people to care, which I imagine they're not.)
Also, I imagine a lot of Republican voters would see the issue as one of a suitably morally acceptable problem to the issue of child pregnancy. Of course, those child and teenage pregancies are predictable results from their reluctance to allow sex ed, and "abstinence only" policies, together with their policies on contraception (think of the Holly Lobby ruling - courtesy of Gorsuch, Trump's appointee to the Supreme Court - which allows employers to exclude contraceptive provision from their employees' health care provision on "moral and religious grounds".)
It's a weird mindset over there in some parts/social strata. Leaving teenage girls in ignorance about how their bodies work, denying them contraception which could help them prevent pregnancy, denying them abortions if they do get pregnant - all fine. Forcing girls into marriage when they do get pregnant - also fine. There is a substantial part of the US voting population who really do think that way, barbaric as it may seem to us.