Explanations welcome

[Lani]

That is because EVERYONE got chemo if their tumor was invasive in the US for years and years (still today!) as it was the standard of care and trials could only ADD treatments onto the standard of care or the doctors could be sued for denying patients "their best hope"
Europeans tried to determine with yearly St. Gallen qualification modifications which ER+ tumors benefited from chemo and ADDED chemo onto the basic standard of care and were unencumbered by malpractice suit considerations.

Endocrine therapy is also cheaper than chemo in general and European countries' national health services had trouble paying for chemos like taxanes and platinum agents.

It was difficult to get US oncologists to look seriously into the possibility that the ONLY benefit premenopausal ER+ patients might be getting from the chemo was the early menopause they were put into ( a result which could be arrived at much more simply)

THere are different cultural attitudes towards oophorectomy, GNRH injections as well as different women's willingness to suffer with symptoms of lack of estrogens...many things come into play.

And this does not even entertain entering into the realm of paranoia about manipulation by/influece of " big pharma"