The main reason i could think of why they remove giving matts after "boom" is because of the "seal shop"
this are just my opinion
1. To encourage other players to keep doing raids. 2. Over supply of rare matts on mp. if players keeps getting their rare matts after item destruction 3. Rare matts value will go dert cheap. actually its very cheap right now it will go much less of a value if we still keep getting our matts back. 4. since all matts are cheap its not really a big issue if we dont get our matts back.