I wonder why we didn't hear about this on mainstream media TV
This is really pretty simple to answer. You live in Texas. The incident occurred in California. This was a limited local news event. It isn't going to get picked up on the national wires unless it is something that is newsworthy enough to command more marketshare than all the other localized events going on all over the the country. So you didn't hear about it on the "mainstream media." The interests covered for the Fresno area are not the interests covered for your location in Texas and so not part of your local mainstream media to cover Fresno or Madera. Wouldn't you hate it if all you heard about was Fresno news in Texas?
If you believe Lott, guns save LOTS of lives everyday like airbags, apparently even more so. Somebody being saved by an airbag doesn't exactly make the news nationally unless maybe it is a celebrity or the crash victim isn't found for a week or two, still clinging to life. Similarly, singular, double, and even multiple domestic murders often don't command much more than local news either. You probably aren't even aware of all the acts of violence that went on in Chicago or New York for the last few nights, are you?
If mainstream media broadcast every positive event that happened in the country along with every negative event, even if those events were just defensive gun shootings and bad guy shootings of good guys, the newscast would last for hours and would get worse when the auto industry started demanding airbag stories be aired.
However, to answer your question more directly, it was covered by the mainstream media. The questions you should have asked is why the OP didn't cite a mainstream media sources or why you didn't google it yourself.
How about the followup?
Of course, the OP wasn't making a case about the media's coverage of shootings and simply cited sources that covered the material to make the point. So that would answer the first question.
The media did its job and covered a local news event.