Because this one was witty, was a decent retelling of an oft-used plot, acted well by all involved, and actually had a continuity that made sense. Decent action scenes, decent injection of humor.
If you have to choose between The Losers or The Expendables for "Friday Night Movie Popcorn Night of Action Movies", I'd say watch The Losers. Or heck, watch both and compare. I think the Losers was much more entertaining because it doesn't have the expectation level at the outset from the cast. And you'll still recognize the cast. True they aren't the historically household names that comprise the cast of the Expendables, but you'll know most of 'em anyhow.
Someone told me they liked The Expendables better because it was straight up B movie done just right. I disagree. I still argue The Expendables set an expectation with the cast they had and never met it and their attempt at a B-action flick was clumsy, disjointed, and an excuse to use lots of pyrotechnics for no reason; whereas The Losers actually just works as a straight-up B movie done right.