The problem is if you have less than a 50% voter turn out, then the whole democratic process can be seen to be illegitimate by the non-voting majority, who are over 50%. Legally, they may very well have a point that they are being run by an undemocratic illegitimate ruling body.
Of course, that would be the claim by the outside political bodies who are anti-democratic who would use this as an example of how democracy is really just a form of authoritative rule.
In order to overcome this legal issue, and also to reflect reality (the 50+% majority just didn't vote, not that they are anti-democratic), compulsory voting overcomes this problem and therefore they can claim to be a legit democratic government.
I believe people actually voted in politicians who were pro-compulsory voting and that is how it spread, especially in Australia.
Bona fide canonical and then some.