A huge part of campaigning is either driving your own turnout (field operations) or suppressing the other guys' turnout (negative ads). Additionally, there's a philosophy of "suppress turnout in general and it will probably help me", which statistically is a good move for Republicans, most of the time.

The argument is that compulsory voting would disincentivize these activities in favor of issues-based campaigning. I agree although I'm not sure I agree strongly enough to favor compulsory voting.