Because once the media crowns a winner (or even just declares a candidate practically eliminated), they tend to underperform their polls even if they haven't publicly conceded.
(When a winner is crowned, it also tends to depress turnout for the whole contest, which has to be a concern for downballot Democrats in California, especially with the top-two primary system for non-Presidential races which actually makes the primary more like the general election in a general + runoff system than a normal primary; while it probably doesn't matter a lot in the statewide races or areas which are Democratic strongholds, it could matter in other places. Actually, in some of the Democratic-favored races it could make the difference between two Democrats advancing in the top-two system and a Republican actually making the cut, so it might matter even there.)
> Clinton seems worried that this coverage could cost her points today.
That's not what you'd usually expect based on history, but its certainly possible (that Clinton supporters could stay home because they don't feel the need to vote, and Sanders supporters show up as a protest to the media coronation), and given the effect it has on interpretation of the results, its the expectation the Clinton campaign would want voice concern about and use to shape media coverage whether or not its a real concern.