Cos 1) It's Monday and a bit of chirpiness might make a difference 2) It's 2016 and (See previous reason) 3) It's so quintessentially American (or at least so mid 20th century American), so youthful and optimistic and chirpy.
I don't even know if I LIKE it that much. I prefer God Only Knows, like everyone else. It's TOO chirpy. But it might make richly ironic music, to soundtrack the muted video footage of a Donald Trump rally.