Why don't the news media report the reasons why migrants come to the UK on boats?

Because that might invoke feelings of empathy & compassion for the human beings risking their lives travelling across the channel.
Or raise awareness to the criminal exploitation migrants suffer to just get a position on the boats. Or the continued exploitation they endure whilst living in the UK. Just ignore what possible reason someone would uproot their lives and travel thousands of miles illegally, I'm sure it was something small & trivial...
No no no... all our world's problems are solely caused by migrants! Certainly not the billionaires exploiting everyone pushing the narrative.
- Or raise awareness to the criminal exploitation migrants suffer to just get a position on the boats
  No, I've seen plenty of reporting on that. Never on the reasons they're trying to get a position in the first place though.
  
  Well then, they don't report because the reason varies from migrant to migrant. Usually there will be a line or two in the report about where and possibly why, but anything past that means they have to interview someone who isn't in a position to be interviewed.

Media gets paid billions to talk shit about migrants.
You bomb their countries and then cry about it when they come to your country.
- Media gets paid billions to talk shit about migrants.
  Eh? By whom?
  
  Everyone who profits off war and softer forms of imperialism. Prime examples are the defense industry, tech industry, and petroleum companies but many others benefit as well.
- Poor, formerly working class, people in the UK have benefited least from the decades of foreign and economic policy that have ultimately caused the migrant crisis, and are being harmed most by it: the market for low-skilled jobs is either much more difficult or impossible, and social services are stretched past breaking. But those people are not crying, they are turning to the far-right in the (probably vain) hope for a solution because everybody else have proved themselves non-credible.
  Even though the current close-to-a-London-a-decade is simply unsustainable, it is absolutely nothing compared to the coming flood of climate refugees. The line will be drawn somewhere. The longer it goes on the stronger the response will ultimately be.
  It is a disaster but that is the reality of the situation.
  For context, the suffering migrants experience now is again nothing compared to the coming harm future migrants will experience due to climate change, and none of us can stop it, because if we don't burn those fossil fuels somebody else is going to. It isn't solvable under capitalism because the cooperation required isn't possible. Write it as an epitaph on human-kind's gravestone: "We didn't possess the wit to overcome the dopamine-driven desire for more, now."
  
  formerly working class
  Just curious why you refer to the poor people you're talking about as "formerly" working class?

Depends on the media source, if you look up a documentary you'll find the reasons stated, but if you look up article headlines it won't have the same balanced explanations (if they mention the root cause at all.)
- I'm talking about news media, to be clear. I presumed "report" would make that clear. I've edited the post title to add the word "news".

Because they can’t drive to an island
- My question is about the media.