Why do the highest 4 on-line sportsbooks have 80-90% of the market share in just about each state it’s authorized in US?

Here is a source of one such situation

I dont actually use their merchandise since I dont gamble. And it’s powerful to seek out data as a result of many of the data on-line is bearish on your complete sector or one of many high 4 who occur to be in that 90% market share. By their stability sheets after which making the case in opposition to that particular firm or sector. If this was some other sector folks would take a look at that high 3 or 4 and name it an oligopoly that would proceed to dominate the market. However for some purpose on-line sports activities betting is one the place folks suppose the 12+ firms with the remaining market share will take over the sector.

Does anybody know why the highest 4 have the 90% of the market share within the first place? That may assist me to know if the competitors with the remaining 10% can actually overtake them. Or perhaps what does the remaining 10% have that places the highest 4 susceptible to being disrupted?