Answer by Sean Burnside:
I’m just repeating other answers to some extent here, but the initial question is flawed in its assumptions. Observing that Slitherin (during the books) was populated by opportunistic and -shall we say- morally questionable students does not equate to them being losers, except purely in the sense that they sided with Voldemort, who lost and therefore they -by extension- lost as well.
That being said, each House had its share of exceptional members as well as members who lacked aptitude in certain capacities. For instances, Slytherin had Severus Snape and Horace Slughorn, both exceptional educators and highly skilled in Potions and Occluency (in Severus’ case). As observed, Crabb and Goyle were poor student, but exceptional in the Dark Arts. The same could be said to some degree about both Harry and Ron. Snape was not completely off the mark when he said that Harry was relatively unexceptional in his classroom. Neither was particularly exceptional nor motivated with their classwork, but their close friendship to Hermione afforded them the advantage of a ready tutor. Yet, both were skilled in their own way and among those ‘skills’ were their courageous attitudes, loyalty and drives.
I think the misconception here is mistaking being opportunistic for being a ‘loser’ or a sociopath. Slytherin House emphasizes ambition and resourcefulness. Both of these traits emphasizes self-reliance over partnerships. While you may make alliances to further your aims, your goal is always your own advancement, usually at the expense of others. Slytherin therefore is a ‘feast or famine’ House: you either excel and your ambition propels you to higher status -such as Snape, Slughorn or most notably Voldemort- or you sink to relative obscurity.
Contrast this with Houses such as Gryffindor or Hufflepuff. Both emphasize a more collegial atmosphere where partnerships bring groups -rather than individuals- to the forefront. This is especially true of Hufflepuff which is almost the antithesis of Slytherin, sacrificing the exceptionality of the individual to the goals of the collective good. Because these Houses actively promote bonds of loyalty, devotion and friendship, they are more likely to excel as a group and be seen collectively as more successful.
In the final analysis, Slytherin are not ‘losers’ or sociopaths, but ARE often betrayed by their inability to manifest their own success or allow their ambition to supersede their morality. Hufflepuff are not ‘clueless’, but rather will often sacrifice their own aims towards the advancement of others. Gryffindor is not populated entirely by winners or by ubermensch, but rather by individuals capable of complementing each other’s strengths and accommodating for their weaknesses.