Because there are other more suitable places for them to expound their POV.
Where - I'm not aware of any other place where non religious people are able to discuss issues of the day from a moral perspective which, presumably (given your desire for TFTD to remain exclusively for religious people) is only for non religious people. If you know of such a programme, do please tell us.
Thought for today caters for a specific audience, (not necessarily believers of a particular faith), but of people that like to hear about different religions in the context of modern day life.
I'd beg to differ - TFTD doesn't have a specific audience at all, given that it is a tiny slot within the Today programme and although broadly at the same time each morning can differ in start time by longer than its entire length. It's audience are people tuned in to the Today programme who will include a mix of religious and non religious people who I would expect reflects the UK demographic pretty closely.
Unfortunately you assume that someone who is religious cannot be a humanist, they can.
No I am not assuming that - but how can you justify the current position where a religious humanist could get a slot on TFTD but a non religious humanist would be banned.
I'm not really interested in some random person trying to explain their whole non religious moral philosophy in just the three minutes they have, remember the faith/ religion of the person sets the POV in a context that a non religious person doesn't have.
It works as a very short programme because the speakers have a context which they can draw on, which is partially familiar to the target audience.
Why would the contributors be 'random' - TFTD has always had regular contributors and all the change would do is start to include interesting and challenging regular contributors who aren't religious but have something interesting to add in the tradition of TFTD.
I disagree on context, non religious people may have just as much context as religious people - the 'golden rule' is just about the broadest moral compass we have and perhaps the group that espouses that most strongly are humanists, including non religious humanists.
And given that the target audience is listeners to the Today programme (who are likely to be about 50% religious, nominally; 50% non religious) why would this audience be particularly familiar with (for example) a Sikh tradition view, and yet not with a broadly secular humanist view. I'd argue that the latter would be much more familiar in context to a much greater proportion of the target audience than the former.