It is an outreach event. The majority of people in developed nations live in progressive cities, and hold center-liberal to outright progressive views.
Now, if you're trying to spread propaganda, good or bad, you want to do it in the most efficient way possible. Just as you wouldn't put a radical Satanist in a government role of promoting birth control, you shouldn't have your PR representative be a man who's against the very role he's supposed to be playing. Both the first instance and the last are counterproductive. An advocate for Satan would be just the ammunition someone against birth control could use perfectly, and a PR representative who's against the inclusion of the people he's supposed to play PR for will naturally alienate.
The John Carlson lecture isn't an opportunity to offer the field cutting edge science, it's an opportunity to present to the rest of the world new findings in climate science, and to inspire others to take up the banner. It is science propaganda, in the best way possible.
Just as you wouldn't invite OJ Simpson to give a lecture on sentencing reform despite his innocence because it would alienate the very people you're trying to convince to rally for sentencing reform, you should aim to have the least-offensive and most-captivating candidate give the annual PR speech. This is meritocracy, which is something that many universities aren't big on (being filled with legacy admissions).
My own views are probably not far from the person who was denied the speech, here, but I understand the value in propaganda, and having someone who would set PR back when most of the point is to captivate the public would be a net-negative, and anti-meritocratic. Despite their viewpoint being probably comparable to my own, I'm aware that PR is delicate, and public advocacy plays a large role in changing public opinion, so they should indeed step aside to let someone more publicly-accepted give the speech.
In short: Don't alienate your audience if your goal is to persuade. Go with a safer bet, always. It's not a great idea to fire controversial people, but if the goal is a good speech for PR, go with someone else. They'll probably appreciate the time to do more research.