One of the joys of the news business is that, no matter how long you’ve been at it, you can still never be certain what will count as news. It’s an unstable category, one that changes shape depending on timing, context and, crucially, who’s deciding.

Take, for example, the unfolding saga of Facebook, Cambridge Analytica and the campaign to leave the EU. It keeps on making big news, the latest development being a decision by the information commissioner to fine Facebook £500,000, the maximum allowed under the current law, for its role in the scandal. The tech giant was found guilty of a double failure: it failed both to protect its users’ information and to be transparent about how that data was being harvested by others.

As the commissioner put it on BBC Radio 4’s Today this morning, elegantly distilling the case against the company: “In 2014 and 2015, the Facebook platform allowed an app … that ended up harvesting 87m profiles of users around the world that was then used by Cambridge Analytica in the 2016 presidential campaign and in the referendum.” (Later the commissioner issued a “clarification”, distinguishing between the US and Brexit cases: “We know that Cambridge Analytica were working on aspects of the US election political process. However we are still looking at allegations that Facebook data was used as part of the referendum campaign, and this forms part of the next phase of our ongoing investigation.”)

Of course, you can find that story playing prominently in the Guardian and elsewhere (including the front page of today’s Financial Times), but the wider story of which it is a part – about not just suspicious but potentially illegal behaviour by pro-Brexit campaigners in the EU referendum – has struggled to break through in the part of the media with the biggest reach, namely the national broadcasters (with the honourable exception of the dogged Channel 4 News) and, centrally, the BBC.

It’s now understood that Vote Leave may have broken electoral law, by violating campaign spending limits. It’s also known that Leave.EU misled MPs about its true connections with Russia: the group’s founder Arron Banks told a parliamentary inquiry he had had “two or three” meetings with the Russian ambassador to London – having long insisted his sole contact with the official was one “boozy lunch” – and on Sunday my Observer colleague, Carole Cadwalladr, revealed that Leave.EU in fact met Russian embassy officials as many as 11 times ahead of and just after the Brexit vote.

Now the BBC has not ignored any of this; indeed, it went first with a leak of the Electoral Commission’s draft findings about Vote Leave spending. But it has not given the story the kind of full-spectrum coverage that it does so well: leading every bulletin, dominating its discussion programmes and interviews, putting it top of the national agenda. It is careful to clock it, to ensure it’s covered, but it hasn’t given it the weight that only the BBC can generate.

Play Video 1:50 Zuckerberg: Facebook believed Cambridge Analytica deleted private data - video

This is not a rare occurrence. The phone hacking revelations of 2011 followed a similar pattern: the Guardian had plugged away for years, mainly ignored, until suddenly the story exploded. Windrush was similar: months of reports by the Guardian’s Amelia Gentleman, until a critical mass was reached and the BBC became fully engaged.

Why has that moment not yet come for this affair? Some will say that the story is too arcane, full of obscure stuff about algorithms and data that news editors fear will leave the average viewer and listener cold. That belief may indeed play a part, along with the lazy assumption in some newsrooms that this is a story to be filed under “tech” rather than “politics”, a niche concern rather than one central to our democratic life.

Perhaps it was deemed to be all a bit conspiracy theory, reportedly involving shady Russians and secret offers of goldmines, coupled with the fact that Banks and Leave.EU look like relatively peripheral figures (even though their spending was colossal and influential). As for Vote Leave, it’s possible that some editors see a potential breach of electoral law as something of a technicality.

All those assumptions would be wrong, of course. People increasingly understand what their data is and value their privacy. It is not just Brexit’s self-styled bad boys who are under suspicion, but the mainstream leave campaign. And if electoral law is eventually found to have been broken, this would be serious.

So as those reasons for holding back fall away, what’s left? One factor may be a belief that, no matter how much skulduggery may have been involved, it did not decide the referendum vote: Brexit was the result of deep-seated shifts in the national mood, not dodgy data or funding, and the media should pay attention to those underlying issues.

That’s a legitimate view and not to be dismissed. But it doesn’t fully explain the broadcaster’s reticence. Surely the heart of the matter is the BBC’s fear of being accused of bias. Can it probe allegations of misconduct by leave if it is not doing the same for remain? Never mind that there is no evidence of a similar scandal on the remain side, just the act of poking so hard at one of the two combatants in what is a bitter national fight chafes against the BBC’s view of its own neutrality.

A story such as Brexit is paralysingly difficult for the BBC, just as Donald Trump has tied much of the established, studiedly impartial US media up in knots. If there are problems, it wants there to be problems on both sides, equally. (For that reason, the fact that the information commissioner is also fining Emma’s Diary, a website aimed at new mothers, for selling the personal data of 1 million people to Labour, may lead to an increase in BBC coverage.) It’s hard for it to report allegations that the Brexit case in the referendum was pushed dishonestly, just as it’s hard for it to report that the Brexit case is unravelling now.

But the BBC needs to put aside those fears and report the facts as they stand, facts which are becoming ever clearer. That, surely, is its primary duty.

• Jonathan Freedland is a Guardian columnist