1. Karin was starting to be noticeably pregnant in November at the SF show. She'll either be ready to pop or just have had the baby around Coachella.
2. The Knife are not popular enough to be be Sahara headliners, and for their show to be entertaining, it requires a massive amount of set up and special stage equipment. It is not reasonable to assume Goldenvoice would allow a non-headliner the leeway to have their stage setup so specifically, especially when you'd have to consider that a real headliner would likely play thereafter, and the timing to tear down the Knife set would overlap with whatever elaborate set the headliner desired.
3. The Knife are finished touring in America for awhile. They're not a touring beast, and they didn't get a delicious taste for the road last year. It was a rarity, and everyone who saw it was quite blessed and lucky.