Where are you getting your information? Tizen OS is not dead. It is well alive and kicking and being improved and Samsung is slowly implementing that unified OS not only for their phones, but for other Samsung products. The main issue for any phone producer is that any OS that they do not own also Android comes at a price - there is no such thing as free OS.

There are also plenty of other mobile phone OS, that are not compatible with every phone and that is the main issue why they are not widespread. Phone that I own is supported not only by Android(and Android forks), but also couple of other mobile phone OS. The trend is that in future there will be more choices for OS, that might satisfy those users, that are currently not happy about Android, but have no other choices at the moment.

There are at least 10 other mobile OS choices - most of them are based on Linux, but current share of those is ~0,1% out of ~6 billion of phones. In total numbers that is only 6 million devices. 6 million device market is a significant number for any company, not to mention, that this number is only playground compared to 1000x larger world market of mobile phones.