Maybe I'm just an insensitive arse.
Err...this implies an intentional action :v
You were curious. Not doing insensitive things intentionally with the full intent and knowledge that, 'oh hey this person will be D:<, LES DO EET'.
Of course, I am aware this is orders of magnitude less significant than actual psychological trauma, but it's the same principle: why should everyone stop using a word/discussing a topic because a small portion of the population finds it upsetting?
To put it in brevity, its 'negative connotations'.
And how you discuss them. And many other factors.
...It'll take quite a lot of text to fully debrief that kind of idea to your question.
But the event by which you're questioning 'why should this stop' == the event prior was discussed in a way that was detrimental. It can be discussed, but not how it was previously discussed (just that people get confused, then wonder what went wrong).
The word by itself doesn't usually upset. The details connected to it which comprise the context is what causes the upset.
If you're still curious: Probably ask someone present there, physically. Like: "{Greetings Professor}, I'm curious about ideas and discussion. Why shouldn't an idea be discussed {situation or reason}", to provide the foundation on the 'Why'. Why is there prohibition. Why is there this 'bad' or averse connection. What makes it sensible to not be discussed? "Could you explain how come some people are ok with the idea being discussed?" towards the 'What'. What makes this idea have such a basis? What makes the topic be prohibited, or to be addressed as 'not discussable'. What makes this word feel bad? Or you could poke at the meaning--as words are the verbal or written manner by which we transfer meaning via thought. "How does this topic upset people?" onto the 'How'. How can a situation of discussing a topic, turn sour? How are there other ways to communicate an idea? How can there be instances, where I said something, and people were ok with it, but other times people hated me for it?
Stuff like that. There's a lot of ideas out there, and I'm not that good of an interpreter(?) because I slip on my English sometimes.