Firefox OS has potential. At least you can load it onto a Nexus device, although it is extremely limited.
I'm surprised there are not more viable OSs.
I'm not surprised. Mobile OSs are faced with an enormous challenge, which is hardware support.
The PC platform (while there are variances) is relatively uniform environment compared to the current crop of SoCs (system on chip) used by phones and tablets. There is hardly any standardization on things like power management, security, and other on-chip peripherals.
Each port for an OS is a lot of effort, so only the most popular OSs (sometimes only one of them) is supported by the manufacturer of the SoC.
Among the would-be competitors, the Jolla team is best equipped to mount a challenge.