I'd say it's in part because of the lack of *off the shelf* variety, and in part because of how damned complicated modern systems now are.
Off the self variety is important, because whilst you're right that there is still a fair bit of variety both of hardware and OSs, the days of being able to walk into a computer store and be faced with a myriad of genuine choice, where most of the machines on display genuinely were different, are just a fading memory. These days you walk into a computer store (assuming you still have one that's open - another aspect of modern computing which differs from those early days of wonder) and whilst you're likely going to be faced with an even larger number of different machines vying for your attention, they'll almost all be nothing more than either a Windows or MacOS box at heart. Unless you put in the effort to go hunt down those alternative platforms, they might as well not exist.
And then there's the issue of complexity. In those early days, computers were simple enough that it was possible to understand every last aspect of how they worked, both at the hardware and the software levels. And this remained, to at least a suitable extent, true as systems started to become more complicated through the 16 and 32-bit eras. You could still reasonably grasp how it all hung together, how it all worked, and what effect doing *this* would have on *that*.
These days, despite all my decades of experience as an electronics and software engineer, I look at a modern PC and think "nope, might as well be a black box". That's not to say I no longer understand *anything* about what they're doing, because fundamentally much of it remains the same as it always was, even if the modern implementations are different. The problem I now have is similar to the problems I now have in my day job dealing with microcontrollers - back when I started on this career path in the mid/late 90s, I was dealing with devices whos *entire* reference manuals required fewer pages than just the chapter relating to *one* of the peripherals on the devices I'm using today. The fundamentals might be more or less the same, but the total amount of knowledge you now need to *fully* understand the entire system, how each part works, how it all interacts with one another, is so significantly higher than it used to be, that it's damn near impossible for one person to hold it all in their head.
And then there's the point that, back then, every single new release brought something genuinely impressive, something that had us going "wow". The pace of change was immense, and it was a thrilling time to be riding that change. These days it's harder to get even remotely interested in the latest news from Intel, nVidia etc about their performance upticks compared with the previous generation of hardware, because whilst things are still improving, the rate of change is generally not only slower, but we're also now at the point where most of us don't really need more than we've already got. Back then, even an extra couple of MHz of CPU power, an extra few hundred KB of disc space, or whatever, was a huge deal not only because of how significant a change it was relative to what we currently had, but also because of how limited we knew we were by what we currently had - we'd take *any* level of performance increase and still be thirsting for more, because we knew we weren't yet at the point where we could do all the stuff we imagined we could be doing.
And sure, some people today still have that feeling - e.g. look at the way those involved in AI are gobbling up as much memory and processing power as they can lay their hands on to facilitate the developments they're making. But for most of us that's no longer the case. Systems are now good enough, there's far less motivation for us to want to spend time and money on pursuing the relentless upgrade paths we once were eager to proceed along as quickly as the manufacturers could churn out new stuff to keep those paths going, and if we do upgrade it's more likely prompted by our current stuff simply being worn out and *needing* to be replaced, than because we've outgrown it and actively *want* to replace it.