Because some developers act on their own consciousness and don't have a slavemaster corporate manager telling them what they need to do or not do.
When one doesn't like any of the available choices yet a new one is born. Can you measure how many v.terminals we have, or how many window managers on X11?
I had a brief opportunity to play around with an AT&T workstation running unix, more like a 386 than any sun/sgi machine that costed as 2 new sportscars. It had a very brief life, despite of the quality of the box, it was pretty useless. Slow as hell windows 95 would run circles around it.
Then out comes DEC/Alpha with Dec's unix, was it ultrix? And in those machines later windows NT was also ported, so it was a testbed between the two worlds. Then RHat CDs rained on us
@jollyrogue @eah