Because most people in the paleo community are hype about ketosis and eat a VLC diet to stay in it. To these people more than 50g carbs per day is not a low-carb diet.
To me, whether your diet is low, moderate, or high in carbohydrate depends solely on the macro-nutrient proportions of your diet. I eat around 400-800 cals (100-200g) of carbs per day. I still consider my diet to be low in carbs because I also eat 1800-2500 cals of fat and 280-480 cals worth of protein.
A lot of us are matching up our daily glucose intake to our presumed daily glycogen depletion. On a low-intensity day, this is about 130g of liver glycogen, but on a heavy weight-lifting day, this can easily swell to 3-400g without saturating total glycogen stores.
i personally dont buy that line in the sand of 50 gms of carbs and ketosis. It needs to be measured. I have found many on a VLC doet not to be in ketosis to turn off gluconeogenesis and it was because of insulinogenic spikes in those who were leptin resistant. If your hormones are not optimal dont assume a thing. And leptin is the most critical one. That is one of the reasons I really give people a hard time about dairy who are not losing weight or hitting plateaus.