I’m assuming this was due to the fact that in late 1990, Marvel started over-producing. Many books appeared twice in a month. The X-universe exploded, and “street” characters proliferated with multiple Punisher comics, the debut of Darkhawk, and others. To say nothing of the vast number of one-shots and “special event” limited series, of which there are too many for me to cover here.
So, Marvel tightened its No-Prize criteria so people would stop telling them all the places the editors screwed up by letting through a miscolor or typo, and now readers had to actually earn the damn thing by doing an act of service in the name of Marvel.
This appeared on the letter page to Hulk #382.