You might mean: a median niche business makes more money than a median startup. Average is much higher than a median for startups because it includes the most

You might mean: a median niche business makes more money than a median startup. Average is much higher than a median for startups because it includes the most successful startups which are by definition have a high growth rate over an extended period of time so they are very big. Niche businesses in this context have a low growth so their average is closer to the median. The growth is constrained at the top in absolute terms (otherwise it would be a startup) so the average is lower than the one for startups.