These things are cyclical. At one time the bulk of 1/72 figures were limited to the scope of US history. The entire Airfix line was focused on Anglo-American history.
Recently, companies like HaT have expanded the 1/72 hobby to cover most major historical epochs. Now that the 3,000 years of human history that preceded the US have been represented, it is time to increase the number of sets focused on US history.
However, I think you complaint is unfounded. Earlier this week HaT posted master figures for two AWI sets. You write as if HaT makes no US sets, but they have been increasing the number of US subjects for several years now.
They make the only complete box of Zouaves for the ACW. They also make WWI infantry and artillery, and they have made US versions of their latest WW2 sets. So, you can't complain that HaT is biased against US history. The facts don't support that statement.
As Mr Buckingham pointed out, HaT decides what to make based on what sells. When people buy every box they can make of a given set, they will make more on that theme. If people don't buy what gets made, they don't make more.