Why not apply Betaderm to the face, underarms, groin, etc.?

Here we go:

Local

Skin atrophy may become evident within one to two months of use and is due to the inhibitory effect of corticosteroids on collagen formation+++ Skin on the face, axilla, and groin appear to be most susceptible to the adverse long-term effects of topical betamethasone. Use of high potency topical corticosteroids on these areas should be minimized or avoided+++

Topical corticosteroid use may impair local immune response rendering the skin more susceptible to infections. Folliculitis has occasionally been reported.

Perioral dermatitis or rosacea-like dermatitis has occurred in patients treated with potent topical corticosteroids. These conditions may flare temporarily upon discontinuation of topical steroids, prompting patients to continue their use. If topical corticosteroids are discontinued, this flare and the initial dermatitis generally resolve over a few weeks.

Worsening of psoriasis has occurred in a few patients.

Local side effects have included burning, itching, dryness, and irritation, especially when applied to denuded skin. Long-term use of topical corticosteroids may result in skin atrophy and thinning, and the development of striae, telangiectasia, subcutaneous hemorrhage, and easy bruising and bleeding. Allergic contact dermatitis has occasionally been reported.

https://www.drugs.com/sfx/betaderm-side-effects.html

I guess not to irritate these parts of the skin that are more sensitive.