Because one team is fighting for Europa League football (on the day they said goodbye their club captain) and the other has been on the beach since March.
It wasn't just the Southampton players who weren't up for it tonight; the fans were silent all game as well. They're a club that's thoroughly done with this season - I'm slightly grateful that Liverpool only clawed one goal back as opposed to the 4-0 drubbing I was expecting them to dole out.
Hassenhutl is clearly off in the summer, the fans and players have lost patience with him. I think he probably thought that chucking a bunch of second-stringers out might catch Liverpool off-guard and might give said second-stringers a chance to prove themselves. Turns out it didn't.
Liverpool fans could just as easily point at us beating Brighton 3-0 the other week and ask why they just rolled over for us when they nicked a point at Anfield. Leicester beat them 1-0 at the King Power pretty soon after we won there. They've dropped 4 points against Chelsea while we've taken maximum points.
That's how football works.