Bras wear out, just like any other piece of clothing. Women who are homeless, in battered-women’s shelter, or unemployed might not be able to afford a new bra. They’re expensive, compared to a shirt. People collect other kinds of clothing for the needy: why not bras. And people donate food and other consumable items for the poor: why not sanitary supplies?
As the lone male in a female-dominated household I know a fact many males do not.
Bras are quite expensive.