Because in hepatocellular jaundice the liver is not working properly, and only a small ammount of conjugated bilirubin goes to the intestine where bacteria convert it to urobilinogen and stercobilinogen, as a result a small ammount of urobilinogen travels through the portal vein to liver. But as you know liver doesn't work as it should, so most of the urobilinogen from the liver doesn't return to the intestine but goes into the blood stream where is increased then, the same story is with the conjugated bilirubin, which also goes from the liver in the blood stream instead in the intestine. So both urobilinogen and conjugated bilirubin are increased in the blood stream and consequently also in the urin, which from the lab results and patients symptoms we say is (hepatocellular) jaundice.

Because in hepatocellular jaundice there will be increased amount of bilirubin in the body it will be excreted in the form of urobilinogen and stercobilinogen through urine.