Why isn't the LA Expo Line a subway?

https://www.youtube.com/watch?v=bMDox81mFP8

Ground was broken today for a 12-mile stretch of light rail from Glendora to Montclair.
https://www.youtube.com/watch?v=dfKIPGwAURw Dec 2017
https://www.youtube.com/watch?v=RbCf1fsP15o