why does it take so long for my hair to grow back after hair loss because of stress? (Photos)

The time it takes to regrow can vary. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

Hair loss related to stress has to run its course.  It will take a good 9 months to a year after the stress issues subside for your hair to appear fuller again.  You have to give it time. 
Hopefully your doctor checked FSH, LH, Free & Total Testosterone, ANA, TSH, T3, T4, Serum Fe, TIBC, Ferritin, CBC, Prolactin, DHEA-sulfate.

I can't confirm your hair loss is due to stress without knowing all your story and seeing your scalp up close. However, if truly is due only to stress and stress is finished, hair density will return in 9-12 months. For many women these shedding events are precursors to androgenetic alopecia so be sure to see your doctor if things don't improve. Other hair loss diagnoses are also possible. Most individuals with hair loss have normal blood tests and the role of family history is generally overemphasized. Keep close follow up

Female hair loss is a difficult analysis and requires many tests, not just thyroid testing. I simply can not write a textbook on the subject on this site, but here’s a list of past posts that may help you understand what you’ve got going on:
Female Genetic Hair Loss Is Different From Male Genetic Hair LossBirth Control Pills and Hair LossWoman With Thinning Hair After Family TragedyDiseases Causing Women’s Hair LossFemale Hair LossThe first step in evaluating hair loss in women, after a detailed history and physical exam, is to rule out any underlying medical causes of hair loss which can be treated. If clinically appropriate, the following disease processes should be considered: anemia, thyroid disease, connective tissue disease, gynecological conditions and emotional stress. Furthermore, over 50% of women going through change of life hormone fluctuations (menopause) experience significant hair loss. It is also important to review the use of medications that may cause hair loss, such as (but not limited to) oral contraceptives, beta-blockers, Vitamin A, thyroid drugs, coumadin, and prednisone. The following laboratory tests have been recommended to rule out the aforementioned medical conditions:
Some common blood tests for female hair loss:
Sex Hormone testsSHBG (Sex Hormone Binding Globulin) used to test status of male hormonesEstradiol is a sex hormoneFSH (Follicle Stimulating Hormone) *not hair follicle but follicle in the ovary*LH (Luteinizing Hormone) is a sex hormoneFree TestosteroneTotal TestosteroneANA (Anti Nuclear Antibody) used to test for Lupus or other autoimmune diseasesTSH (Thyroid Stimulating Hormone) used to test for hyper or hypo-thyroid diseaseTest Iron statusTIBC (Total Iron Binding Capacity)FerritinIron