Two primary reasons:
1. We didn't do a Matrix because we didn't know a way to do a matrix that would allow for simultaneous presses, which seemed important for this board. A matrix would probably be a more efficient system, which actually feeds into the second reason.
2. It was super cheap and relatively straight forward to do it the way that we did. I'm using Elegoo Mega 2650's, which are $12 a piece and with a docking station attachment I also don't have to soder. I have a pile of I/O expanders I bought to do it the other way, but this seemed a touch more efficient . For a larger scale project I might go the I/O expander route, but with a prototype I didn't want to necessarily soder and then, upon finding that I'd done it wrong, de-soder and then soder again. I have steady hands, but I don't know if I have "soder 100 tiny wires to tiny boards" steady.