The main reason would be I think the demographic that musing has currently..
See, one thing happens when a new user first comes to this platform. They see quite a few things are easier here. you can ask questions and eaisly get upvotes by writing answers. So because of that a tendency flux occurs that makes the users write answers...
But the users currently writing answers here, most of them do not have enough sp to upvote a post and reward it with much. This inability makes them think if I upvote it wouldn't matter and so they don't upvote. And if they do upvote they only do to answers.
Another portion of users that have a little bit more sp upvotes answers but their daily limit reaches to max easily.. Because they also upvotes post in steemit and the answers here.
And now when it comes to the users who can upvote with enough juice to help the community they do it by uupvoting the answers because it positively meketizes musing and thus helps the community. Atleast they think so I think.. for example you can see that there are more answers here than questions. This can change if musing is delegated more sp to reward both questions and answer writers in general. I think in a few more months musing will reach that mile stone.