That letter was written for a specific case. A yeshiva, which had only bought C”Y milk for years, was thinking of switching. R’ Moshe urged them not to. This psak was meant ONLY for the yeshiva, not for the general public.
You’re mistaking it with the one printed in ????? ???.
That one is in ??”? ?’ ???? ?”?. In that ?????, written about yeshivas, he specifically mentions that for reasons of chinuch, they should be ????? despite the fact that over the course of the year it’s a large expense, and the financial situation in the yeshivas is tight.
This one was written to ?’ ????? ?????????; that one to ?’ ???? ??? ??????.