Why is that? People with deadly viruses aren't insulted that we call malevolent programs computer viruses. Equating my laptop with a human would be insulting, but I don't think that was done here.
Because real cancer kills people. Wikimedia spending more money is not going to result in anyone actually dying. Calling it a cancer trivializes the pain and suffering of people with actual cancer and their families.
I am the author of the Wikipedia essay "Wikipedia has Cancer". For the record, I am myself a cancer survivor, and I have received multiple emails from other Wikipedians who have cancer saying that they have no problem with my essay title.
I've been very fortunate never to have had cancer, and to only ever know one person who did. But she'd dead now so I can't ask her for her opinion.
I think we're just going to have to agree to disagree about this.