The organizers of a conference generally need to put restrictions on the amount of content presenters can communicate to their audience. For example, talks cannot go over time, a poster has specified dimensions, conference papers have page limits and sometimes limits on font sizes and page margins, etc. This is to ensure that attendees have a pleasant experience and are not overwhelmed by an unreasonable amount of information by presenters eager to capture more than their fair share of the audience’s attention.
(This eagerness is amplified in competitive situations in which getting more of the audience’s time and attention can increase a presenter’s chances of, say, winning the Best Poster Award, or of generally improving their standing in the community. So these restrictions are also about fairness and ensuring that presenters don’t game the system to gain an unfair advantage over other presenters.)
As for the rule about hyperlinks, I don’t have a strong opinion about whether it’s a good rule, but it seems likely that it’s at least motivated by these sorts of considerations.