Why do the top 4 online sportsbooks have 80-90% of the market share in pretty much every state it is legal in US?

Here is a source of one such situation

I dont really use their products since I dont gamble. And it is tough to find info because most of the info online is bearish on the entire sector or one of the top 4 who happen to be in that 90% market share. By looking at their balance sheets and then making the case against that specific company or sector. If this was any other sector people would look at that top 3 or 4 and call it an oligopoly that could continue to dominate the market. But for some reason online sports betting is one where people think the 12+ companies with the remaining market share will take over the sector.

Does anyone know why the top 4 have the 90% of the market share in the first place? That might help me to understand if the competition with the remaining 10% can really overtake them. Or maybe what does the remaining 10% have that puts the top four at risk of being disrupted?