Our Constitution protects free speech. Free speech isn't free if the government can force you to speak. Dictatorships force people to vote even if they;re the only candidate running. They do this just to legitimize their power and position.
Why should someone be forced to legitimize one or the other of two people if neither is capable in your mind? Not voting tells the parties they should do better in picking qualified people if they want your vote. Twisting the voter's arm and forcing them to vote is taking away their freedom to express their opinion.
Okay! It seems I haven't explained the situation in Australia clearly enough. Of course a government cannot literally force a person to speak or vote, just as they cannot force a person to drive a car below a particular speed limit on a road.
Breaking the speed limit is a more serious offense and the fine is greater. If the offense is repeated too often, one's driver's licence is cancelled. However, as far as I know, there
is a type of freedom to refrain from voting by simply paying the fine, or turning up and making an invalid or informal vote which won't be counted. Elections don't happen often, so a $50 fine is no big deal. If one can't afford the $50 fine, then that's reason enough to vote for the socialist Labour Party, or someone like Bernie Sanders.

I should also point out that Australian citizens do not elect the 'President' or Prime Minister. The country is divided into electorates, and each electorate always has a significant number of candidates belonging to different parties, and sometimes candidates independent of all parties, all competing for the one seat, in either the House of Representatives or the Senate, that represents a particular electorate. It's extremely unlikely there could be only one candidate in an electorate. Voters also have to show preferences on the ballot paper for the other candidates. If there are 4 candidates, for example, one has to rate each candidate from 1st preference to 4th preference.
Voting in Australia is considered both a right and a
responsibility.