Aliens wins more often becouse playing humans require more teamplay. With teamplay, teammates covering when Youre reloading, some humans toegether shooting at the same alien from attacking alien team You can really take advantage of humans ranged weapons. I've seen some games with good human teams playing toegether and covering each other and (almost) no alien can pass or reach them. On popular servers humans loose more often becouse everybody plays on his own, what works better for aliens.
EDIT: Moreover, when I (or someone else) just managed to persuade my teammates to go out and attack instead of camp (humans team) most of such games was won by humans. Humans in S1 aren't so defenceless, You can kill dragoon even with rifle

. And light armour + bat pack + pulse rifle can make single human very effective

. Problems come with assholes camping in their base until Aliens S3

. Although smart use of BS + chain can also harm some tyrs.