Why would the conference organizers ban the inclusion of new links (e.g. to datasets or repositories) in the text of the rebuttal?
The underlying reason might be that rebuttals should not present new data/results which were not included in the original submission.
Presenting new data/results in a rebuttal is problematic because it increases the review load: The new data might require some detailed explanations of how they were obtained and how they add to the overall work. Effectively, the reviewers have to discuss two versions of each paper (without and with the new results). The organizers might want to avoid that, given that the review load at good conferences is normally already quite substantial.
Of course, one could still allow links if they don't present new data/results. But this raises the question why these links were not included in the original paper. Potentially there have been previous experiences according to which the risk of misusing links to refer to new data/results is seen as too large.