What would be the reason that estrogen depletion symptoms are not cured by estrogen in a woman who is post hysterectomy?

Because the ovaries do not make just estrogen. The ovaries actually make androgens and enzymes, and the hormones needed are created in the blood by the precursors created by the ovaries. So you end up with estradiol (most potent human form of estrogen), progesterone, and still have some of the intermediary androgens used to make the estrogen and progesterone.

What you are calling a hysterectomy here might actually be a "complete hysterectomy," or an "oophorosalpingohysterectomy." That is when not only the uterus is removed, but the ovaries and Fallopian tubes are too. That is what a veterinarian does when they spay an animal. They remove everything. So when the ovaries are removed, women lose not only estrogen, but the other hormones as well. In addition, they would likely need more estrogen and doctors are too scared to give enough estrogen to help them. They often don't realize that surgical removal requires more estrogen than natural menopause does.

Then you run into issues with bio-availability and hormone ratiOS. Human females have 3 main estrogens circulating - 17 beta estradiol (e2), estrone (e1), and estriol (E3). Hormone replacement replaces only one since presumably, the body can convert between them as needed. Whether women in this age bracket who have surgical menopause can convert between them is another issue. While compounding pharmacies can make you a ratio similar to what is found in other women, the FDA doesn't approve the addition of estriol, nor do they ban it at this point either. Estriol seems to do well enough on its own at relieving hot flashes without causing breast symptoms. But if given in high dosages, estriol can break down into an estrogen compound that may be implicated in certain cancers. Nobody has submitted estriol or a combination drug to the FDA for evaluation, so the medical profession cannot legally make any beneficial claims for it.