The arc of the moral universe is long, but it bends toward realpolitik.

It's because the IBM PC was released in 1981, and over the next several years, as prices came down, steadily overran pretty much every other competing platform and took all the variety out of the market from a historical perspective. If you're interested in 40-year-old computers you could have a collection with a z80 machine, a 68k, a 6502, a tms9918a, or an 8088, and that's just variety in CPU architecture, and just some of the popular ones. Everything else was the wild west, too. Go backward too many years from there, though and the home and small business computer manufacturing industry just isn't as big, and specimens become an order of magnitude harder to find that aren't just glorified calculators. You have to put up a lot of cash comparatively to get the fun part of the hobby of owning and using them yourself over just reading about them, which you can do for any machine in any era, plus they're harder to service. If you're collecting 20 years later, though, things had gotten so much more standardized and developed that it feels almost like a different hobby. Most all of what you'll buy will be some variant of the HP versus Emachines dichotony: either an expensive IBM PC Compatible using all the common standards, with a high-tier x86 that mostly does what they all do but faster, and maybe an add-in specialty card, or a cheap IBM PC Compatible with some of the common standards, some things shaved off for cost, a low-tier x86 processor that's just more frustrating than your fast one, and a motherboard covered in cheap components that you have to solder in replacements for before it even works again.

I've painted a bit of a skewed picture here, but not by much. You can still collect later computers, and people do, but it's understandable that most people are drawn to the "cambrian explosion" of the whole line of history, no? Variety is the spice of life, and plenty its staple food to be spiced.