Silent Assassin was a new concept (Codename 47 didn’t come out for consoles) that excited people and got them to buy it and try it out, it was well regarded at the time and people enjoyed it.
Contracts was often criticized for just being more of the same, a rehash, so not as many people were interested. It also only came out 2 years after Silent Assassin.
Blood Money came out 2 years after Contracts, 4 years after Silent Assassin, same deal, not as many people were interested, the engine looked to be the exact same, to the naked eye, to the eye that isn’t a fanatic of these games, it’s just more of the same. In other words, only the people that really really really liked the Hitman games kept with the franchise, whereas people who tried out the Silent Assasin game kind of moved on. That is why those two games sold worse.
Absolution came with its brand new engine, 6 years after a Hitman game had been released, it was again a new thing that got the interest of the average audience.
That is why those two games sold well and the other two games sold badly, not because of your subjective opinions on the gameplay changes.
People just aren’t as well informed as you think they are, they don’t know all the kinks, nooks and crannies of the gameplay elements of each new iteration of the series. They sure as hell wouldn’t have been able to tell those if they didn’t buy the game and played it themselves. How can Blood Money sell worse because of its gameplay elements if the people that didn’t buy the game didn’t even know of said gameplay elements? Because they didn’t buy the game, because they weren’t interested. In other words: Because those weren’t the reasons they didn’t buy the game.