Because plan 9 was developed naturally in the Lab as an evolution of Unix research.
Inferno on the other hand was a blunt force attempt by Lucent to leapfrog Sun by halting Plan 9 development for a whole year forcing the team to instead work on Inferno. It was a commercial project to produce a product which was in stark contrast to the more academic/research oriented development of Plan 9.
Unfortunately, this hamfisted attempt by Lucent left Inferno in a state of Limbo, pun intended. The target was more along the lines of embedded systems like set top boxes and early PDA's. The Dis VM also suffers from hard coded 32bit pointer magic which means it only builds on 32bit plan 9 or in a 32bit chroot under a unix-like for hosted mode. And finally, the Charon web browser has a bug that crashes the Dis VM. I have yet to see it boot on bare metal hardware in its entirety. These reasons are why Plan 9 gets all the love, it works out of the box on 386/amd64 and arm32/arm64 (Rpi, Zynq, etc)
Though not all is lost. Someone is working on getting it to build on 64bit 9front and OpenBSD, though the code is very much a WIP (if interested, hop into #cat-v on freenode and ask about the Inferno 64 bit port). It's not completely dead. Patches welcome :-)
I believe that info came from a plan 9 user who interned at Bell Labs in the 00's. Though honestly you can get much better answers from the source by posting to the 9fans mailing list. Charles of Vita Nova, current owner of the Inferno code is a regular on that list as are many of the original Bell Labs folks.
