Compared to the number of online casinos that operate outside of the US, which was more than 3,000 the last time I checked, the few sites that will welcome players from America are limited in what software they can use. Because of licencing restrictions imposed by the actual software manufacturer, casinos wishing to take American players have to choose from a small list of available games makers, which includes the more well known TopGame, RTG, BetSoft and Rival Gaming, and lesser known brands such as WGS Technology. Or they can choose to develop their own games.
Games manufacturers such as MicroGaming do not allow casinos that use their games to operate in the USA.
There is a good chance that this will change over time as more and more gaming companies decide to enter the market, but unless a big name provider, such as NetEnt, decides to enter the market, USA players are likely to have to continue with less well known products. That does not mean that the quality of the games available is not good enough - though in some cases it does mean exactly that - but it certainly means that the choice and variety of games available to players from America will continue to be limited for a while yet.
Finally, you should note that though a particular casino may be listed under a certain brand of software, if that casino is using games from many different suppliers, then they will most likely not have all of a brands titles available. They may choose to offer table games from one software company, and slots games from another one entirely.