But for him it was not an important failure; (16)
Whether or not our speaker admits it, the introduction of an evaluative term like "important" suggests that our friendly farmer might not have been oblivious to Icarus's fall. He's not like the oblivious dogs or horses. He's a human – which means he had a choice. And apparently he made a bad one.
the sun shone As it had to (17-18)
Here's the thing about nature: it doesn't have much of a choice. The natural world goes on. Auden emphasizes this in order to underscore that some folks (that would be us, guys) have a bit more choice than, say, the sun.
the expensive delicate ship […] had somewhere to get to (18-20)
See? Here's a clear choice. Sure, it's a bad one. But it's definitely decisive. Money and profits over life? Why not? Of course, it doesn't sound quite that nasty the way that Auden describes it…but that's the basic point.