When implementing the decoding in silicon, each bit you want to compare requires one or more transistors; but no transistors are required for the "don't care" bits.
Since the decoder is implemented in a PLA-style matrix, you don't save any space by omitting transistors. So that's not a motivation.
Random thing: chip transistor counts usually count "transistor sites" rather than physical transistors, so the omitted transistors still show up in the transistor counts.
Random thing: chip transistor counts usually count "transistor sites" rather than physical transistors, so the omitted transistors still show up in the transistor counts.