I think the default sorting for Steam reviews is by how helpful a review was.

Looking through some of the positive and negative reviews, it seems like the positive reviews are fairly short and don't say much about the game, while the negative reviews are longer and the reviewers do mention what they thought did and didn't work about the game, so the longer reviews that have a negative opinion end up being the most helpful. Just telling someone you liked something isn't going to cut it for everyone.

I can only guess that everyone who wanted to write a more in-depth positive review was just so keen to get back to playing the game that they cut their review short ;)