I think part of the reason might be that over time, more and more companies that had early success in the genre, as well as those that are new to it or whatever - realize it is popular and are trying to figure out what makes it tick - what are the things that make people so infatuated to play it. But I think they lose focus of making a fun game that is gamer oriented for the sake of fun and other factors that matter the most to gamers, and they hire psychologists on their teams to find out how to maximize addictability traits in the games which in turn will lead to players playing longer and dragging their friends in, and more or less taking advantage of people by praying on human frailty. This leads to massive amounts of DLC, microtransactions, various game mechanics targetting people's obsessive compulsivity and other psychological tactics. It works on many people and the games make money often, but the real gamers end up thinking WTF??? That's not fun! It is nothing like the good old games I remember and still want to play! Why don't they make new OLD games!?!?
Sadly, it is because those games get broken down into a formula that then gets attempted to optimize it to produce profit, and profitability and addiciton outweigh all the fun things and consumer friendly things that made the games like Diablo etc. so exciting and fresh and new. Sure, Diablo had some of that addiction psychology in it too, and most games do to some extent, but there are degrees of that, and if it is balanced just right you have a hit on your hands that everyone tends to put on the top shelf. When the balance tilts towards gaming the gamer to milk money out of them and pray on weakness, those who are stronger minded about those gimmicks are more likely to see through them and judge the newer games as crap.
That's the crux of my theory anyway. I'm sure there's a lot more to it than that though, but I think the offshoot of what I'm suggesting is what has led to the nickel and diming type DLC, microtransactions, and various other nonsense.
Another aspect I find with many games is that more and more they want to make the games cross platform to PC and consoles, and most of the time they either develop the title for console first then port to PC, or PC first and port to console, or a balance - but their mind is always thinking "this has to work on console", and so the game's design ends up having certain elements built into it that cater to consoles and console gamers so that they don't have to maintain two separate forks of the product.
For example, if you were to try to have a game like Star Wars: X-Wing Alliance coming to market in 2016 for the first time it would not be the same game remotely. It would be severely dumbed down and more of an arcade style shooter. Why? Because a PC keyboard has 100+ keys on it with modifiers and you can have 300 ship functions assigned to various keys, to ALT-this and CTRL-that etc. as the keyboard has tonnes of input controls, but on a console you have basiclaly an XBox 360 controller with what 10 buttons? So they have to design the core game play around the limitations of what a gamepad can do even if PC gamers will be using a keyboard, so that they can maintain one codebase and maximize code sharing as well as having a uniform game experience between the platforms so it doesn't seem like two different games. The PC games thus are often dumbed down from what they'd be if they were PC exclusives and they have this unmistakeable feel of consolitis that is anywhere from weak to strong. This is true even if the game is primarily a PC game that gets ported to console and not just games that are console that get ported back to PC. I friggen hate this. Even the Witcher 3 had some signs of this although it was on the lighter side of things and you could kind of ignore some of it and reconfigure around a bit of it, and they addressed more of it in future game patches eventually (such as not being able to rebind keys for example).
Today if a game requires 90 individual keys on the keyboard to do something, that game is simply not coming out for Xbox or PS4 and some companies don't want to do that, so we just wont get those kind of games from those companies.
There are many examples like these. I know your original question was about hack n slash games, but I think the reason behind the answer is actually true for most game genres and not just hack n slash. When profitability climbs higher than creativity and building games that the developers would want to play themselves, then we all lose. What's funny is that often games fail because of this and if they developed an honest game focused on the gamer instead it probably would have sold better, but they never learn...