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This has got to be the only industry where the products you use and your religion are one in the same.
For which you can blame on two things, IMO.
1. Computer and OS vendors have historically kept all their eggs in different baskets - thus choosing another hardware vendor meant choosing another OS, other software and perhaps even different peripherals. In fact DEC used to support more than one proprietary OS on one architecture - the PDP-11 - and no more than two PDP architectures were ever compatible with each other (e.g. PDP-6 and -10). By contrast, even though people argue about cars, choosing Ford over Toyota this time round has never meant ditching your Toyota-shaped bottom for a Ford-shaped one. Even when DOS and Windows PC's came along, you still couldn't buy more than one or two architectures (depending on how you count) to run them on - at first by design, and then by default.
2. AT&T are the only people who (grudgingly or accidentally) created an open platform - which, until the advert of Linux, STILL got proprietarized. IBM, of course, accidentally did it in hardware too.
The only reason that Linux (even more, Linux or BSD) is the only game in town for many people who run FOSS is that it's the only mature/maturing technology based on a sort-of-open one. To the casual user, even certain aspects of BeOS/Zeta/Haiku look more "Unixy" than (say) "Windowsy" or original(-y?!).