The nightcap is cheap and low effort. All you really need are two pieces of scrap fabric to sew together. A knit beanie has to be, well, knit. Now we have machine/robotic knitting, but hand knitting consumes a huge amount of time and energy. The nightcap clearly came about at a time when clothing was straight up made in the home, often from scrounged fabric. Nowadays the higher quality beanie is the obvious choice. It’s snug to your head, can be purchased at many different price points, is manufactured using cheap outsourced labor halfway around the world and can be at your door step 12 hours after a mouse click. I don’t think I’ve ever seen someone unironically wear one of those floppy night caps in my life unless it was part of a cheap children’s novelty pajama set or costume.