Because Chittappa’s family could not buy festival clothes that year, and in a family what one member has is shared with the others.
The story gives the chain of events plainly:
Chittappa lost his job; Chitti is a homemaker → the family had financial difficulties
Acchamma told Shalini’s father about it → the elders spoke to each other
Onam was coming → new clothes are part of the festival for everyone
So Shalini’s parents bought clothes for Chittappa, Chitti and Chinni as well
The money had to stretch further → Shalini got a simple cotton dress instead of the silk one she expected
Acchamma’s own explanation, in the book’s words, is the answer to this question: “this is how families support each other and share what they have.”
Why this counts as more than kindness: the chapter has already named the values a family teaches — dāna (giving), sevā (service) and tyāga (sacrifice). Shalini’s parents practised dāna, and Shalini herself practised tyāga without being asked to. Notice also how it was done: nobody was made to feel small. The clothes were bought quietly while shopping for everyone, so Chittappa’s family shared the festival as equals rather than receiving charity. That is why the last line of the story is about happiness, not loss: “She was happy that everyone could get some new clothes.”