"Why should it not be mentioned people are happy to point out when people are killing themselves through drugs/drink/tobacco but not through weight issues"
There have been programmes on weight issues, including one where some overweight people had chances to meet others who had done nothing about themselves being overweight. I cannot recall the name but do remember an American woman of some 60 stone who needed carers to roll her and give her a wash, as she appeared confined to bed (and had died before the programme went out). Another was a man who had lost one of his legs (don't recall the reason, but it was weight related).
It's one thing to go onto a show where your lifestyle is under the spotlight and another where some women, albeit clinically obese, are being asked to appear to model clothes for a daytime TV show.
In the first case, the health issues, along with your likely early death, will be pointed out, and quite upsetting for a participant, but they are doing it to get a 'reality check' and push themselves to change.
In the second case, if there was a chance those women would be given a nasty, judgemental, description, covering all the wrongs of them being overweight, not only would they be (understandably) upset - they were there to model some clothes - but any agency that put them forward would be angry at the This Morning show, and it would likely be blacklisted by many other agencies.
While I can understand your point (and did not watch the show), and accept that anyone who is overweight needs to be conscious and try to reduce their weight, it's far from the time or place - given the intention of their planned coverage - to be "blunt" about weight.
"curvy" might be too positive for you, but they were considering the feelings of their models, the audience, and viewers, and not someone judging the term as too sympathetic or even glorifying rather than realistic.
(FWIW, I'm overweight and was thinking just today, after a neighbour had been in hospital for a week - "where the hell would I get some pyjamas in case I ever needed to go back into hospital" - though I have lost 2 inches around my waist in the past 3-4 weeks).