They don't handle well, which for many keepers is a big part of the appeal of keeping snakes. Sure, they don't bite, but they're flighty and move like greased lightning, especially compared to a python. Even just taking it out into the yard for a picture leaves you at pretty high risk of it getting away. They're long, slender and more likely to escape than a python.
They're pains to feed. They'll eat fish... generally... which means you need to constantly buy live fish, which is expensive, a hassle, and they don't always eat them well. You can feed them frogs which is legally and ethically questionable as well as potentially risky with parasites etc. Lizards are the other option (when I've kept them I've mostly used the Asian Pest Gecko). Many people are uncomfortable giving them live feed, and it's always less convenient than something which enthusiastically takes from tongs something you can buy from a shop and store without maintenance.
They're not as easy to house as pythons.
They have much higher metabolisms than pythons and if anything is wrong or if they're not fed frequently, they'll rapidly lose condition.
They're generally more sensitive than pythons.
They're not as easy to breed as pythons.
Despite all this, a lot of people continually insist that it's a bit of a mystery as to why they're so rarely kept, and despite being obviously true, a lot of the above is disputed by many people.
They're definitely super cool animals, they're very active, perhaps the most active Australian snake, and make great display animals. They come in many colour forms, some extremely striking and beautiful, many people try to work with them but almost everyone stops bothering before long.