In a democracy, the government must represent the people. Everyone, not just those that had showed up to vote. Only true democracy happens if all citizens are obligated to vote to allow to have their interests represented in the democratic government. Only representing the interests of voters would undermine the principle of democracy.
If someone is not motivated to vote they are almost certainly not motivated to research and understand the issues. Forcing them to vote will only lead to masses of people making uninformed decisions. I once had a co-worker who said he wasn't voting because he didn't know enough about politics to make a good decision. I admired his honesty.
There are many different reasons for people not to vote. They might not like or support any of the people that are contending. Also, forcing someone to vote will cause many people to make an uninformed decision. This unbalances the polls and the election will not always reflect the results of the majority of the informed voters.