As I said in another post ... this recent primary election in Washington, only 54% of eligible people voted. And we were "sold" on mail-in voting because it would increase voter participation.
So, here's a reason why voting should not be mandatory.
What if that 46% of people who choose not to vote were forced to - so many of them said "You're forcing me to vote? Fine, take this." and voted in such a way that they thought would screw up government.
Don't think it could happen? I use the draft troops in VietNam as an example. It isn't talked about a lot, but there were many who did just enough not to get court martialed or killed.
There are nations other than Australia where voting is mandatory - but most of them only do it as a PR move. "Look at our voter turnout. 90% voted for {x}." (Of course, candidate {x} was sponsored by the state or ruling party and the vote is a sham.)