Because history[0][1][2][3]. The timelines (and license, in the case of VMS/OpenVMS -- even to this day) didn't make it inevitable, but certainly paved the way for Linux (although one or more of the BSDs could have filled that niche instead, had a fully unencumbered[5][PDF] BSD become available sooner) to have the level of adoption it ended up having.

VMS has always been proprietary, despite the name, and is still an expensive[4] option for commercial use

[0] https://en.wikipedia.org/wiki/History_of_the_Berkeley_Softwa...

[1] https://en.wikipedia.org/wiki/Minix

[2] https://en.wikipedia.org/wiki/History_of_Linux

[3] https://en.wikipedia.org/wiki/OpenVMS

[4] https://vmssoftware.com/products/licenses/

[5] https://dn721902.ca.archive.org/0/items/att_unix_licenses_19...