Well for a start, low voter turnout isn't just a cause but an effect. It’s bad because it demonstrates that something about the political system isn't engaging the people it impacts. Exactly what or why will vary but it generally can't be a good sign.
Low voter turnout can cause (further) democratic deficit. It makes it easy for extreme minorities to impose their views or even for outright electoral fraud to happen. In more general terms, it further distances the people from the government, making it easier for leaders to disregard general public opinion or general needs.
You seem to be making the leap of logic to mandatory voting but there is a lot of scope between people choosing not to vote and them being forced to, indeed, I don't support any kind of mandatory voting without the kind of information and schemes that would seek to encourage more people to choose to vote (and in an informed manner) anyway. Get that kind of thing right and mandatory voting becomes something of a moot point.
Of course politicians and the partisan politicos don't want more people to vote for many of these reasons. It's much easier for them to maintain the status-quo with a small number of partisan voters and a large body of uniformed non-voters. And I'd argue that the very reason politicians oppose something is a good reason to do it
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