"Card" is two characters less than "shield", and has a long history in computing denoting a circuit board that is plugged into another circuit board. (Whereas "daughter" is usually not redundant in "daughterboard", it can almost always be dropped from "daughtercard".)
Examples: network card; disk controller card; video card; SD card.
"Shield" is such an entrenched term in electronics, from long before computing, that it's completely stupid to try give it another meaning. It's also not from some specialized sub-domain whose rare terminology can be reused; shielding is employed in all kinds of devices.