Probably because it's a long, tedious, repetitive task. Brushing teeth only take a minute or two, maybe three if you're getting really thorough, whereas flossing thoroughly or consistently adds much more time to that. At least that's my excuse
My old dentist suggested keeping floss wherever you sit to relax (his example was wherever you sit to watch TV). That way, you can spend as much time as you need to floss thoroughly, without it feeling like flossing is taking up a bunch of time out of your day. It's not a perfect solution, but it has been helping me.
The little pick things you can buy are a game-changer if you're struggling to make it a regular thing. Probably shit for the environment, but they make it easier to get into flossing regularly. Especially if you struggle with the normal flossing wire stuff at all.