Minority is a noun that is often used as an adjective -- like many nouns in English.
A noun that is used like an adjective is called an attributive noun. Many words are modified in this way. I'm not sure that there's a reason for this; it's simply one of the characteristics of the English language. Here are a few examples:
In an earlier post on this subject, my colleague Paul Wood wrote:
In the verbal illustration that you cite (“24-hour turnaround time on most orders”), turnaround is used as an attributive noun. An attributive noun is a noun which modifies another noun without a linking verb (as apple in “apple pie,” bank in “bank building,” or turnaround in “turnaround time”).
Attributive nouns are generally distinguishable from adjectives used attributively in that:
(a) they cannot be used predicatively ("the time was turnaround" is impossible),
(b) they cannot be premodified by very ("a very turnaround time" is impossible),
(c) they do not take comparative or superlative forms ("turnarounder time" and "the most turnaround time" are impossible),
(d) they cannot be made gradable by modifying intensifiers ("so turnaround time" and "highly turnaround time" are impossible).
There are some adjectives which also do not meet the above criteria. Such adjectives are distinguishable from attributive nouns in that they can only function as modifiers for nouns or noun phrases.