why?

I'm interested to know, WHY is PC so open? what led to that?

Many vendors, because that means you need specs and that in turn allows for interoperability

Agreement IBM had to make with the DoJ/etc in the 80s to open the PC platform to avoid antitrust prosecution. That was the key event.

You might be interested in the IBM PC compatible and Wintel wikipedia pages. This is a super high level timeline, but it is more interesting to get into the detail.

At a high level, the IBM PC platform were very well documented & sold well, to the effect of producing tons of software and peripherals add-ons ("PC Compatibles"). This led some other computer companies to reverse engineer the proprietary IBM BIOS, allowing them to run the same software and use the same peripherals. Because these were clean room reimplementations, IBM didn't have a legal case to prevent their sale.

Fast forward a bit, IBM's attempt at a new, closed platform, PS/2, flopped. People wanted their more open hardware. Windows became dominate enough that all the demand was for x86 based hardware that could run Windows. Microsoft was happy to work with many vendors.

The PC is very open today, but Apple survived. Atari ST and Amiga probably survived longer than you think as well.

Because Microsoft commodified their complement in the 1980s to break the back of IBM.